Understanding Exposure

H

hazmo

Suspended / Banned
Messages
30
Edit My Images
Yes
As advised by various members of the forum I've been studiously reading a book called "Understanding Exposure" to gain a better understanding on the mechanics of photgraphy. There is a section in the book which refers to "depth of field" on digital cameras when using zoom lenses in that it states that it is difficult to get a shallow depth of field as F2.8, F4 etc are actually multiplied up by a factor(?) to make them F11 or F16 which consequently increases the depth of field. Or have I read this wrong?

Can anyone try and explain (simple terms please) as I am still trying to decide on what equipment to buy and thought I'd learn a bit before I blew the kids inheritance and this has left me somewhat confused.
 
I've just finished reading that also, and I thought it was something to do with the crop factor on the P&S cams. For instance my Z2 has about a 6x crop factor, therefore f3 on that, would be the equivalent of f18 on a 35mm camera, if I understood it correctly :thinking:
 
Hazmo, what page number(s) are you looking at? Let me know and I'll have a look and try to explain what he is saying, if I can.

By the way, you can play around with some numbers in this online DOF calculator to see how DOF varies with different combinations of camera body, focal length, aperture and subject distance.

http://www.dofmaster.com/dofjs.html

If we plug some numbers into that calculator as follows....

Canon 40D body, 100mm focal length, f/2.8, subject at 10m, the total DOF is 1.07m and it extends from 9.49m to 10.6m away.

Canon 5D body, 100mm focal length, f/2.8, subject at 10m, the total DOF is 1.69m and it extends from 9.23m to 10.9m away.

So, all things being equal except the camera body, you get more DOF on a full frame body than you do on a crop body. But, the crop factor for a 40D is 1.6X, making the 100mm focal length the equivalent of 160mm on the full frame body, so let's plug in those numbers....

Canon 5D body, 160mm focal length, f/2.8, subject at 10m, the total DOF is 0.65m and it extends from 9.68m to 10.3m away.

So, now, once we have adjusted the focal length multiplier to make the field of view equal we find that the DOF advantage is reversed, and now the crop body has more DOF than the full frame body. In order to achieve the same DOF on the full frame body you would need an aperture of approx f/4.6.

Putting that the other way round, to achieve the DOF you had at f/2.8 with the full frame camera, you would need an aperture on the crop body camera of approx f/1.8. There are no f/1.8 zoom lenses available in the Canon world, so what you can achieve with a full frame camera and a zoom lens at f/2.8, in terms of narrow DOF, you cannot achieve with a crop camera and a zoom lens. You would need an f/1.8 prime.
 
Thanks for the replies. I've left the book at home (doh!) so I'll post the page numbers this evening.
 
Hazmo, my copy is handily sitting on my desk. It's page 46.
 
OK, I've read the page and it is simple enough if you know the topic in general. He is basically saying two things....

1. When you shoot with a camera with a small sensor, whether that be a point and shoot camera or a DSLR with a crop sensor there is a multiplication factor you use to convert the actual focal length of the lens to the equivalent focal length, in terms of field of view, on a full frame 35mm camera. Canon entry level DSLRs (XXXD and also XXD) have a crop factor of 1.6X. The Canon 1D series has a crop factor of 1.3X and the Canon 1Ds series has a full sized sensor with a crop factor of 1X. Nikon DX bodies have a crop factor of 1.5X. Point and shoot cameras have a much higher crop factor, because their sensors are much smaller, so the crop factor might be something like 4X or so, but it does vary.

2. Due to the different sensor sizes, and the resulting variations in absolute focal lengths to match the corresponding focal lengths on a "standard" full frame 35mm camera, the DOF calculations for these cameras give very different results compared to the full frame equivalents. Basically, with a full frame camera it is quite easy to create a shallow DOF and quite hard to create a really deep DOF. In contrast, point and shoots struggle badly to yield a shallow DOF and yet can easily manage to produce a huge DOF. Crop body DSLRs fall somewhere in between.

You typically want a great DOF when shooting landscapes and needing everything in the shot to be in focus. The same goes for macro photography, when the close distances involved really make it hard to get much DOF at all. Conversely, you would typically want a shallow DOF when making a portrait, to emphasise your subject while blurring the background to oblivion. That's true for any situation where you want to use selective focus to highlight a specific detail and make other vanish. Shallow DOF is also great when you need to make something disappear, like mesh fencing in front of an animal at the zoo, or a car at the race track. Full frame DSLRs are brilliant for such shots, while compacts are hopeless and crop body DSLRs fall somewhere inthe middle, but nearer to full frame DSLRs I think.

I don't know about individual conversion factors for apertures and DOF for each type of camera or size of sensor, but the general principles are clear....

- Small sensor cameras can easily give you massive DOF
- Small sensor cameras struggle to give you really shallow DOF (it can't really be done at all)
- Large sensor cameras can easily give you really shallow DOF (with fast lenses)
- Large sensor cameras struggle to give you really massive DOF (it can be done but it is hard and can lead to other complications, like slow shutter speeds)
- Crop sensor DSLR fall between the two extremes, good but not great at both ends of the spectrum. With very fast glass (usually expensive!) they can get a shallow enough DOF for most needs in that department.
 
Thanks for the explanation which I will print off and stick in the book for reference.

Just to clarify, if I buy a 40D with a 17-55 zoom lens (or similar) then in real terms that translates to a 17 x 1.6 - 55 x 1.6 focal length. Is that right? Would it then be impossible to take photos of flowers or birds close up with only the subject matter in focus and the background in soft focus? I suppose a lot depends how far you are away from the subject and how far the background is. Maybe it's the wrong lens in the first place!

Too many variables I think!!

Thanks again all.
 
I have a 40D and 17-55 lens (amongst many others). Yes, you have the calculation right - 17-55 is equivalent to a 27.2-88 lens in the full frame world.

I doubt you'll get very close to birds, with that lens, unless they are pets :)

But you are on the right track about DOF and the importance of subject distance and background distance.

Here are some real quick snaps of a flower I just took with my 30D and 17-55 lens at f/2.8 and 55mm. I used the 30D because that camera already had the lens mounted, but for this example the results are identical to a 40D.

All shots were handheld. The first was focused manually at the closest focus distance possible for the lens and then I simply moved the camera back and forth till the flower was sharp. This setup gives the maximum background blur posible for the scene as a whole and this camera and lens. The distance to the front of the lens was only around 10cm. I somehow doubt that most birds would let you approach that close. For bird stuff you really need more like 400mm, and often that is not enough.

20080910_162830_3039_LR.jpg


Two and three were taken from somewhere around 1m away, give or take, (clearly one example is a lot bigger than the other but I didn't measure the distance or pay close attention) but at the same aperture and focal length, just to give a better idea of the scene as a whole.

20080910_162852_3040_LR.jpg
20080910_162924_3041_LR.jpg
 
You are on the right track but Understanding crop factors and DOF etc although great knowledge can just add to the confusion at the beginning.

Its something to keep referring to as you understand what each compnant of the 'shot' is made up from.

I have learnt that, yes, a 50m lens on a 1.6 crop factor body is essentially the same as a 80mm lens on full frame.

However - dont get caught up in the numbers. You still get what you see in the viewfinder. And essentially - F1.8 will be much shallower DOF than F8 with the subject in the same place. Keep these settings and move the subject and the DOF will change.

Practice with differing DOF and focal length combinations withount moving the subject distance.

Then do the same moving the subject. Make a mental note of the change in effect. You could even store the examples on your pc noting down all the factores and revew evey now and again as a refresher.

Also, dont forget to use the DOF preview button. Can be extremely useful.

DOF Master when used correclty, takes into account the crop factor of the camera and is a handy tool. But again, if sharp landscapes are a must look into hyperfocal distance. I carry a small laminated sheet with the hyperfocal distances for variouse lens and aperture combinations.

i did the same as you. I read and re-read trying to understand the theory without looking at the effects on tha actual camera/shot. Once i just played around and experimented the theory became a lot clearer and now i stop and think more aboutthe effect im trying to creat and the best dof required.

Experimane with the many variables, review the results and learn from them. Then the theory and the practice come together and it gradually becomes clearer.....im still ad the slightly murky ohase though!!!
 
A great thread! I never realised DoF was affected by sensor size. I knew compacts struggled with shallow DoF, but thought that was just something to do with their small size or something. :shrug:

Don't mean to hijack the thread here, but i think it relates...

Does a full frame sensor also need a less open aperture in low light. What I mean is, where I might need 1/60th f/2.8 would a full frame only need 1/60th f/4.5 to expose the same? Or does it need the same, because there's a bigger area that needs exposing? Or am I completely missing the mark and need to read a book too?
 
Thanks for the explanation. Great pics by the way. I see what you mean about the bird thing. If you go to a 400mm lens are the apertures likely to be smaller and if so by the time you factor in the crop multiplier would it be possible to take a picture of a bird but similar in composition to the first (subject focus all else blurred) but from maybe 10m away? Or is that just not possible with a crop sensor DSLR and a long focal length zoom lens?

Maybe I should just enrol on a photography course!!!
 
hazmo said:
Maybe I should just enrol on a photography course!!!
Click to expand...

hazmo said:
I am still trying to decide on what equipment to buy and thought I'd learn a bit before I blew the kids inheritance
Click to expand...

Just buy (i'm sure someone in the know would recommend what to buy)... there's no substitute for actually getting out and taking pics.
 
I'm sure you're right. Just trying to make up my mind whether to go for Canon 40D or Nikon D300 but that's a whole other debate!!

Time to put theory into practice.

Appreciate all the comments and advice.
 
"I never realised DoF was affected by sensor size."
It's not. There is so much confusion brought about because of this desire to compare lenses to a full frame field of view. It's really not worth it, at all.

Saying that the sensor size has an effect on the DoF is exactly the same as saying cutting out the middle of a print with scissors will change the DoF.

Things like the size of the pixels can have an effect on how the image enlarges but that's heading off into another area. Do a little web search on circles of confusion if you want to get clued up.

"but thought that was just something to do with their small size or something."

It is exactly that. :thumbs:

The reason that compacts give this huge DoF is that they have teeny little sensors and to give them a zoom that covers the field of we are used to in a walk about camera/lens, they use lenses like a 6mm to 18mm. If you've used a really wide lens on your camera you'll know that the super wide lenses have DoF, even wide open, that almost covers the front glass on the lens all the way to infinity.
 
An exposure is an exposure is an exposure. If a compact needs 1/60 at f/2.8 and 100 ISO for a correct exposure then so will every other compact, crop DSLR, full frame DSLR, medium format or large format camera.

Of course, if you kept identical exposure settings for all the formats then the DOF would vary hugely between them, but the exposure itself would be identical.

You can also freely alter one component of the exposure so long as you match that change in one or both of the other components. It's all an "exposure triangle" that balances out for any given amount/strength of light hitting the scene.

e.g. You could double the shutter speed to 1/120 (actually it would be 1/125) - that's -1 stop of exposure adjustment - and that would make your exposure half as bright. To compensate you could double your ISO, from 100 to 200 - thus adding +1 stop - and that would bring your exposure up to the right level again. Alternatively instead of adjusting the ISO you could open up the aperture a bit more, from f/2.8 to f/2 - +1 stop again - and achieve the exact same exposure once again. There are many many ways to skin the exposure cat.

As you build your skills, what you need to remember is how these different variables affect the final outcome of the scene. You choose your shutter speed to control the impression of motion/movement in the subject/scene (and also camera shake). You choose an aperture that is appropriate to control your DOF. You choose an ISO that allows you to use both the shutter speed and aperture that you would like, and still get an accurate exposure.

Sometimes your needs/wishes place excessive demands on the ISO setting, which may make you rethink your other choices, and perhaps you will have to compromise a bit on shutter speed, aperture or both. Tackling these matters is what begins to separate photographs from snapshots. When you factor in and master the choice of focal length, perspective, composition, timing and lighting and choice of subject then you've pretty well got it sewn up.
 
dazzajl, What you say is technically correct, from a certain standpoint, but for practical purposes, for the likes of Joe public, including me, to take the same picture from the same spot, and include the same subject/scene at exactly the same size, in percentage terms (not absolute physical size) within the frame being captured, and at the same exposure settings (aperture especially), you will need a different focal length to achieve an equivalent image from each camera. As a result of trying to match everything else in the picture you will find that you get a different DOF from one camera compared to the other.

An example with some made up numbers, but basically sound for the purposes of comparison......

I want to shoot my mate Clive, standing on the other side of the room, about 10' away. He is 6' tall and I want him to fill the frame from top to bottom, with maybe just an inch to spare above and below. On my 40D I find I need a focal length of 50mm to do that. I don't want anything but him to be in focus so I open up as wide as I can to f/1.8.

I now want to shoot an equivalent photograph of Clive with my 1Ds3 full frame camera, from the same position, 10' away, in order to give me the exact same perspective. I find that to keep Clive the same size in the frame I need an 80mm lens, so that's what I use, and I set it to f/1.8 as well.

So I now have an identical photograph from each camera. Clive fills the frame nicely in both and everything was shot with the same aperture and other exposure settings. If I was to print the two shots, which would have Clive looking sharper? Let's look at how the DOF compares....

40D - 50mm, f/1.8, 10' gives a DOF of 0.81' - perfect to keep all of Clive in focus from his belly to his ears :)
1Ds3 - 80mm, f/1.8, 10' gives a DOF of 0.5' - and Clive's belly is out of focus

So for the same photograph, which is, after all, the things that people actually shoot, you get less DOF from the full frame sensor than the crop sensor, at the same aperture settings.

Your theoretical argument, about cropping out the centre of a print, is not really what photography is about. Nobody shoots to create an absolute size of image on a sensor (say 2mm x 2mm), regardless of the sensor size. People don't go around shooting with the same focal length for the same subject when they swap between camera formats. They change the focal length to suit the format. They shoot to fill the sensor with their subject to the extent that they desire, whatever that sensor size may be. That is the real world of photography. So for practical purposes a smaller sensor does give you a greater DOF, for a given aperture, when you try to create the exact same photograph with each camera.

It doesn't have to be Clive either. Anything will do - a car, a boat, a building, a group of friends, a scenic landscape, a macro shot. Whatever you are shooting, if you want the subject to fit the frame in the same way you will get more DOF at a given aperture with a smaller sensor. What you will find with the larger sensor is that you will have to stop down more in order to achieve an identical photo and that will then mean you have to slow your shutter speed and/or increase your ISO to compensate. That may have implications for camera shake, or subject blur, or noise in the image. Whoever said photography was simple?
 
hazmo said:
Thanks for the explanation. Great pics by the way. I see what you mean about the bird thing. If you go to a 400mm lens are the apertures likely to be smaller and if so by the time you factor in the crop multiplier would it be possible to take a picture of a bird but similar in composition to the first (subject focus all else blurred) but from maybe 10m away? Or is that just not possible with a crop sensor DSLR and a long focal length zoom lens?

Maybe I should just enrol on a photography course!!!
Click to expand...

If you want small birds, like robins, tits etc. to be of a worthwhile size then you really will need a long lens, like 400mm or more, or a very good hide to conceal yourself. For larger birds you may get away with shorter focal length. Here's one, slightly cropped, taken with my 40D and 100-400mm lens at 100mm and f/8....

20071202_094845_2027_DPP.JPG


and another - same lens, same settings, different crop....

20071202_095259_2061_DPP.JPG


Here's one with my 40D and 100-400 at 400mm, plus a Kenko 1.4X teleconverter, making 560mm in all, or 896mm if you convert that to the 35mm equivalent. The lens was set to f/5.6 but the teleconverter knocks that down 1 stop to f/8...

20080530_120204_4580_LR.jpg


Last one - 40D, 200mm, f/2.8....

2735302512_a6a4effb57_o.jpg


Generally you will get smaller apertures with longer focal length lenses, or you will pay an absolute mint for the lens. The aperture f/stop value is calculated very simply by dividing the focal length by the actual physical diameter of the aperture. So the longer the focal length the larger the aperture has to be in order to maintain the same f/stop value. A 500mm lens with an f/2.8 aperture would need the aperture to be 500mm/2.8 = 179mm. Now the aperture is where the aperture blades are, buried somewhere within the lens. You' need spacea round the aperture to store the aperture blades, the mechanism for operating them and the barrel casing for the lens itself. So you're probably talking about a lens that is at least 20cm across somewhere in the middle. The front optic would be way bigger than that. That's a lot of glass, a lot of weight, a lot of engineering and a lot of cost. Not much market for one of those - so the cost goes up even higher.

If you want to estimate what sort of DOF you get with long (zoom) lenses then punch some numbers into the DOF calculator. My 100-400 goes from f/4.5 at the wide end to f/5.6 at the long end. Imagine a bird/creature at 20m distance and stick some numbers into the cruncher.
 
What you say is technically correct, from a certain standpoint
Click to expand...

That would be from the standpoint that those are the facts. :lol:

This is a discussion that works so much better down at the pub than it does over the web. Than again, what doesn't? ;)

I do see what you're saying and all your examples about clive and his belly stand. BUT.... the DoF is still a factor of lens focal length and not sensor size. I know that the outcome is the same but if you're going to really understand what's going on here, sensor size dictates focal length and it's focal length that's the factor on DoF here. :)
 
There are two variables at work here, the circle of confusion of the camera and the actual focal length of the lens.

To compare the DoF between a FF camera and a crop camera both need to be adjusted so that you compare apples and apples - in this case our apples are the field of view.
i.e. using a 60mm lens as an example on a 1.6 crop body, you need to compare this with the same image which would require a 96mm lens on a FF body to get an equivalent field of view.

The effect of these two variables, in relation to the crop camera is:
for the larger CoC on the FF camera to increase the DoF
for the longer (actual) focal length on the FF camera to reduce the DoF
The effect of focal length is greater the CoC and the effect is to reduce the DoF for a FF camera "when comparing two shots with the same field of view" - think of this as zooming in on a FF camera to get the same image taken with the same lens from the same position on a crop camera, longer zoom gives shallower DoF.

If you do not care about field of view and just switch bodies on the same 60mm lens (you will of course get a different picture), the DoF will be greater on the FF camera - because the effective focal length has fallen.
 
superb pub thread,

Where shall we all ameet lol!!!

Essentially, the answer is EXPERIMENT.

test both cameras your thinking of. Feel them, hold them, look at the functions and where the buttons are placed. They are a much of a muchness and you will never get the right answer from anyone as each to their own. it really is personal preference ( I have a 40D and think its cracking, however the next man will swear by Nikon).

As for lenses. From a beginners point of view it all depends on how much you want to spend. If your buying a great camera you want decent lenses. All alenses are pretty decent from the cheapo sigmas to megabucks L range. Only you know how much you want to spend. If i could go back to the beginning and buy my gear again this is what i would save up for

40D / 24-70 f2.8 L (or 70 - 200 L for wildlife)

For a beginner with enough cash this would more suffice as you learnt the ropes.

Then experiment with the DOF. It really does come down to experience and practice. But most of all...have fun.
 
gpc1, personally I would not put a 24-70 on a 40D body. It is big, heavy, expensive and really not wide enough when mounted to a crop body camera to work as a general purpose all round lens. Remember that 24-70 really works as though it is a 38-112, which really barely offer a wide option at all. Think that if you're shooting indoors, for example, and want a group shot, you can only back away so far before you hit a wall. If your lens is not wide enough you are stuffed. If it is too wide you can always crop a bit, but you can't do the reverse.

The natural choice for general purpose shooting normally starts around 17mm or 18mm. Look at the kit lenses - 18-55 or 17-85. If you want a quality walkabout lens for the 40D then I'd recommend a 17-55 f/2.8 IS. You gain the benefit of IS and a far more useful wide end, in my opinion. If you wnat quality on a budget then look at the Tamron 17-50 f/2.8. You'll lose IS and AF performance will be slower, but IQ will be close.

Here are examples I shot this morning, of my dining room. I was backed into one corner as far as I could go and took a shot at 17mm. I then guesstimated 24mm (and got 23mm) and took another shot, to illustrate how much of the scene you lose when going from 17mm to 24mm. Obviously 24mm would be slightly worse than my 23mm example.

17mm
20080911_102352_3045_LR.jpg


23mm
20080911_102412_3046_LR.jpg


As you will see, 17mm would easily get a table of dinner guests into shot. 24mm would not.

Another thing about IS vs non-IS. Suppose you are shooting in dim conditions, say a church or museum, and either tripods are not allowed or you don't have one with you. IS will probably get you the shot where a non-IS shot would not.

As for the long lens, I have the 70-200 f/2.8 IS and also the 100-400. Again, purely in my opinion, the 100-400 makes the better general purpose wildlife lens. I often find myself at 400mm and wishing for more. The 100-400 makes a fabulous "zoo" lens. The 70-200 (any of them) is a cracking lens, but really not the best all-round choice for wildlife - especially birds.

The 70-200 gets a good workout as a portrait lens, and for shooting indoors in poor light (weddings), where the 100-400 would be useless. It is also wonderful for wildlife when the wildlife is large or close. Very often, though, wildlife is small and distant and 200mm just won't cut it.
 
Intersting point,

Rather than Hi jack the thread I have sent you a pm
 
tdodd said:
dazzajl, What you say is technically correct, from a certain standpoint, but for practical purposes, for the likes of Joe public . . .

SNIP
. . . .
That may have implications for camera shake, or subject blur, or noise in the image. Whoever said photography was simple?
Click to expand...

Excellent post . . . .
As you explained fully, sensor size DOES have an influence on depth of field, in a roundabout way . . .
 
Can someone tell me what IS means? I am still learning and get lost with the acronyms and abbreviations. Really useful information by the way and my thanks to all contributors.
 
hazmo said:
Can someone tell me what IS means? I am still learning and get lost with the acronyms and abbreviations. Really useful information by the way and my thanks to all contributors.
Click to expand...

There was a rule of thumb in 35mm days, which said that to avoid camera shake you needed to use a shutter speed of (1/focal length) or faster. e.g. if you were using a lens at 200mm focal length you would need to use a shutter speed of 1/200 or faster. You still had to take care, when holding the camera and pressing the shutter button, but that was a nice easy guideline to work with. Some people could shoot steadily at slower speeds, while some would need even faster speeds, especially after a caffeine rush :)

Now that many people have cropped sensor cameras, which effectively magnify the image, and the effect of any movement at the camera end, the rule needed updating to be shutter speed faster than (1/(focal length x crop factor)). With a crop factor of 1.6X, to reduce/avoid camera shake you would need a shutter speed of 1/320 or faster for that 200mm lens.

If the light conditions are poor it may not always be easy to achieve those high shutter speeds, especially if you wanted to avoid noise from increasing the ISO too high, or wanted to stop down a bit for some extra DOF.

What IS gives you is the ability to hand hold a lens at shutter speeds 2, 3 or even 4 stops slower than the recommended guidance. So, for that lens at 200mm, normally requiring a shutter speed of 1/320 to hold steadily, IS would allow you to shoot as follows....

2 stop IS allows shutter speed of 1/80
3 stop IS allows shutter speed of 1/40
4 stop IS allows shutter speed of 1/20 (which is amazing!)

The only issue to watch for is that IS only helps with camera shake and will do nothing to stop your subject blurring from its motion, so sometimes you still need fast shutter speeds anyway (sports, for example). The ideal world is to have IS and fast glass in a lens and then you have the best of both worlds - but you pay for the pleasure.
 
tdodd

Superb explanation:thumbs:. I never really understood this either....now i do!!!
And seeing as my mates call me shaky (especially the morning after the night bfore) I could do with IS on my body!!! LOL
forever learning
 
You must log in or register to reply here.
Back
Top