Focal length what a pain in the butt !

[BADGER.BRAD]

Hello all,

I'm constantly messing with cameras with different size sensors and film formats and this Focal length thing is a total pain in the backside. I generally like a really wide angle lens as most of my photos seem to use that end of the lens. But trying to work out how wide a given lens is on a camera is a total pain surely there's a better way !

 

[woof woof]

I find the crop factor a help but I suppose it only helps if you know what it is and can relate it to FF or something else

Anyway. That's what I do... relate everything to FF and then it seems to make sense to me. For example yesterday I used my Panasonic GM5 with a 20mm f1.7 and that's a x2 crop system so in my mind I think when I use it at f4 it's like a 40mm lens on FF at f8. That works for me

Sony APS-C is a x1.5 crop so a 20mm at f4 on that would be like a 30mm at f6 on FF.

 

[AndrewFlannigan]

That's because manufacturers don't make enough of a fuss about field of view.

In the days of film, it was often the case that lenses were described by the field of view as well as the focal length. Hence the Leica 21mm Super Angulon was 92°, the 35mm Summicron was 64° and the 50mm Elmar was 45°.

The Nikonians have a table for full frame, DX and CX lenses here: https://www.nikonians.org/reviews/fov-tables
There's an attempt to deal with field of view for M43 cameras here: https://www.dpreview.com/forums/thread/4518917

 

[StephenM]

I more or less regularly use 7 different format sizes from 35mm up to 10x8. I use the "standard" focal length for each format, and use simple mental arithmetic to work out coverage for different focal lengths. Format shapes can make this less simple - 35mm at a 2:3 ratio is at the opposite end of the scale from the 1:1 of 6x6 roll film - but this method works for me. If I had to use the frankly confusing (to me) ad man's method of "crop factors" I doubt I could cope!

 

[JimDun]

Yeah, I'm also struggling to get my head round this. My Canon camera has an APS-C sensor which according to the manual means the effective focal length is about 1.6 x what's shown on the lens. Presumably that applies to my old lenses from my 35mm camera as well if I use them on the Canon, so they won't behave the way they used to.

 

[woof woof]

Yeah, I'm also struggling to get my head round this. My Canon camera has an APS-C sensor which according to the manual means the effective focal length is about 1.6 x what's shown on the lens. Presumably that applies to my old lenses from my 35mm camera as well if I use them on the Canon, so they won't behave the way they used to presumably.

Yes. If you use a 50mm on FF and get used to it using it on APS-C will feel strange as the focal length will effectively be 50 x 1.6, 80mm. This is one reason why I always think it a bit strange when people recommend a "nifty 50" to APS-C users.

This really phased me when I got my first DSLR all those years ago, a Canon 300D. I hadn't researched it at all and I was puzzled that 28mm didn't seem like 28mm any more and of course on APS-C it was more like 45mm.

 

[woof woof]

I more or less regularly use 7 different format sizes from 35mm up to 10x8. I use the "standard" focal length for each format, and use simple mental arithmetic to work out coverage for different focal lengths. Format shapes can make this less simple - 35mm at a 2:3 ratio is at the opposite end of the scale from the 1:1 of 6x6 roll film - but this method works for me. If I had to use the frankly confusing (to me) ad man's method of "crop factors" I doubt I could cope!

Having read a couple of responses I suppose we're not all the same

Having only used 35mm film cameras for years and then moving to APS-C DSLR's I eventually found the crop factor to be a real help and that's still the way I think today when using my MFT x2 crop Panasonic cameras. I find it helps to use the crop factor to equate both the focal length and aperture (for DoF and wider image quality) to FF. For eg. I took my Panasonic MFT GM5 out yesterday with a 20mm lens and I knew if I used it at f4 I'd get near enough what I'd expect from FF at 40mm and f8. That seems simple enough and it works well enough for me.

 

[JimDun]

@woof woof I didn't twig the effective aperture would also change, but obvious now it's pointed out as less of the image falls on the sensor.

EDIT: so for depth of field calcs I need to use the effective focal length, not the actual focal length?

 

[BADGER.BRAD]

I have film cameras for 110 , APS C, H & P 35MM and 120 roll film, and 3 different sensor sizes on digital, I normally do as you say Alan convert to 135 format but it would be much more convenient if there was just one standard.

 

[davholla]

Yes. If you use a 50mm on FF and get used to it using it on APS-C will feel strange as the focal length will effectively be 50 x 1.6, 80mm. This is one reason why I always think it a bit strange when people recommend a "nifty 50" to APS-C users.

This really phased me when I got my first DSLR all those years ago, a Canon 300D. I hadn't researched it at all and I was puzzled that 28mm didn't seem like 28mm any more and of course on APS-C it was more like 45mm.

I thought it was because that combination makes it very good for portraits?

 

[woof woof]

@woof woof I didn't twig the effective aperture would also change, but obvious now it's pointed out as less of the image falls on the sensor.

EDIT: so for depth of field calcs I need to use the effective focal length, not the actual focal length?

You can look up DoF tables for APS-C and see if they help but I don't bother now, I just remember to multiply the focal length and aperture by the crop factor.

So, if I'm used to 35mm on FF and I want about the same on APS-C I know I need to do the math... 35/1.6=21mm, so 20mm is near enough. For DoF it's the same math... f8/1.6=f5. So, if I want the same sort of look I'd get from FF at 35mm and f8 on APS-C I need to use a 20mm lens at f5.

That seems to work for me for focal length and DoF and IMHO it's a good enough guide for image quality too if we believe that the bigger the sensor the better the image quality and therefore using the smaller format at a wider aperture may keep the ISO down or the shutter speed up and that might help.

An example from yesterday taken with a 20mm f1.7 at f1.7 on a x2 crop system...

That doesn't look like 20mm and f1.7 to me but it could well look like 40mm at f3.4 on FF

BB.
Sorry to take this slightly off thread but I hope it helps

 

[Deleted member 95743]

When you use a camera with a "crop" sensor, all that is happening is that the camera records a smaller portion of the image circle projected by the lens.

 

[woof woof]

I thought it was because that combination makes it very good for portraits?

I don't know if people recommend 50mm to APS-C users for that reason. I could be wrong but I did think they recommend it out of habit forgetting that APS-C changes the field of view.

 

[woof woof]

When you use a camera with a "crop" sensor, all that is happening is that the camera records a smaller portion of the image circle projected by the lens.

Yes, but it has the effect of creating a zoom effect and as a result you may well stand in a different position.

For example if you frame a shot using FF and a 50mm lens you may be 10m from your subject. Frame the same shot with a 25mm lens on MFT and you'll be what distance from your subject? 10m.

 

[woof woof]

Sorry to take this maybe even further but this might help Jim.

These are three DoF tables I had on my pc. The first is for the FF Canon 5D, the next is the x1.6 crop Canon 20D and the last is for a x2 Micro Four Thirds.

PS.
So from the above if we use f8 and 10ft as guides, 50mm on FF at f8 is about the same as 35mm at f5.6 on APS-C (the focal lengths and apertures don't quite match here but are maybe near enough) and 25mm at f4 on MFT.

 

[davholla]

I don't know if people recommend 50mm to APS-C users for that reason. I could be wrong but I did think they recommend it out of habit forgetting that APS-C changes the field of view.

The weekly imogen uses either Canon 550 with 50 mm or 6D with 85mm lens for glamour type photos so I assume that they want that affect (not all photos are suitable so be careful).

snapshotsx | Flickr

www.flickr.com

 

[woof woof]

The weekly imogen uses either Canon 550 with 50 mm or 6D with 85mm lens for glamour type photos so I assume that they want that affect (not all photos are suitable so be careful).

snapshotsx | Flickr

www.flickr.com

Good stuff. In the past many people used 50mm lenses as do it all lenses thinking it gave a human vision like perspective and some people still believe that to this day. However when you mount a 50mm on APS-C it then acts like a short telephoto and the perspective and the results it then gives differ from those given by a 50mm on a FF 35mm camera because to get the same FoV you have to stand further back when using the APS-C and 50mm combination.

 

[StephenM]

For the same degree of enlargement, depth of field is independent of focal length. With a smaller format, the degree of enlargement is greater, and so the degree of blur of out of focus areas increases, which gives the apparent effect of reduced depth of field. As L320Rio said above, a smaller format merely crops the image, and has no other effect.

 

[The Duke of Bork]

Isn't this whole thing putting the cart before the horse?

Take the camera you have, with the focal lengths available to you, and frame up the image, perspective and depth of field the way you want it. If you can't get the effect you're after, you need a different lens, or body, or both.

Or learn to use what you have.

 

[Kell]

Yes. If you use a 50mm on FF and get used to it using it on APS-C will feel strange as the focal length will effectively be 50 x 1.6, 80mm. This is one reason why I always think it a bit strange when people recommend a "nifty 50" to APS-C users.

This really phased me when I got my first DSLR all those years ago, a Canon 300D. I hadn't researched it at all and I was puzzled that 28mm didn't seem like 28mm any more and of course on APS-C it was more like 45mm.

I got caught out by exactly this.

My first DSLR was a Canon 600D. Everyone (here) said that I should definitely get a nifty fifty.

So I did. And it was a great addition, but it was way too 'zoomed' for indoor shots at parties for example. Often found i couldn't get far enough back to get more than one person in.

In the end I got a 30mm which is better on that camera, and latterly a 24mm pancake lens which has the additional benefit of being tiny.

In fact, it wasn't until I got a 5D that I realised my 24-70 lens was actually a wide angle.

 

[woof woof]

For the same degree of enlargement, depth of field is independent of focal length. With a smaller format, the degree of enlargement is greater, and so the degree of blur of out of focus areas increases, which gives the apparent effect of reduced depth of field. As L320Rio said above, a smaller format merely crops the image, and has no other effect.

Yup... but... If we stand at point A and take a picture of something at point B with FF and APS-C both fitted with 50mm lenses at f8 (or any other combination of focal lengths and apertures) we end up with two pictures which look quite different if printed at any size we can imagine. If we then move back and increase your camera to subject distance the APS-C picture will still look different to the FF picture. This is why I personally think the crop factor is helpful as it's an easy way of equating something with something else. In my case equating APS-C or MFT to something I'm more used to, FF.

Maybe people who've grown up with APS-C don't need to know crop factors but they may then have a few confused moments if they pick up a FF or MFT camera.

 

[woof woof]

I got caught out by exactly this.

My first DSLR was a Canon 600D. Everyone (here) said that I should definitely get a nifty fifty.

So I did. And it was a great addition, but it was way too 'zoomed' for indoor shots at parties for example. Often found i couldn't get far enough back to get more than one person in.

In the end I got a 30mm which is better on that camera, and latterly a 24mm pancake lens which has the additional benefit of being tiny.

In fact, it wasn't until I got a 5D that I realised my 24-70 lens was actually a wide angle.

Exactly why I think it's important to be clear in what we're saying.

Another thing which often raises an eyebrow for me is people recommending a 24-70mm to APS-C users. That could be a great choice but I think it's important to be clear that when using a 24-70mm lens on APS-C you lose the wide end you'd have with a 18-50mm.

 

[woof woof]

Isn't this whole thing putting the cart before the horse?

Take the camera you have, with the focal lengths available to you, and frame up the image, perspective and depth of field the way you want it. If you can't get the effect you're after, you need a different lens, or body, or both.

Or learn to use what you have.

But instead of putting up and making do or spending time and money buying new lenses or bodies or both wouldn't it be better if we understood a bit more and were thus able to make better buying decisions?

 

[Kell]

I must admit, my lens strategy (if you can call it that) in the early days was hopelessly naïve.

It was not based on any research into what would suit me best on a crop-sensor camera.

And TBH, I'm still confused as to whether or not my little Canon (G5 X) which is sold as a f/1.8-2.8 lens is an equivalent aperture or not.

It's marketed as a 24-100 equivalent, but is actually a 1" sensor with 8.8-36.8mm zoom.

Does that mean the quoted apertures need to be multiplied by 2.71 to get their FF equivalent?

 

[woof woof]

I must admit, my lens strategy (if you can call it that) in the early days was hopelessly naïve.

It was not based on any research into what would suit me best on a crop-sensor camera.

And TBH, I'm still confused as to whether or not my little Canon (G5 X) which is sold as a f/1.8-2.8 lens is an equivalent aperture or not.

It's marketed as a 24-100 equivalent, but is actually a 1" sensor with 8.8-36.8mm zoom.

Does that mean the quoted apertures need to be multiplied by 2.71 to get their FF equivalent?

I think, I'm pretty sure, the aperture will be the real aperture

I have a Panasonic 1" compact which reports the focal length in FF terms as you look at the EVF and in the exif it gives you the real focal length and the equivalent FF focal length.

 

[StephenM]

Yup... but... If we stand at point A and take a picture of something at point B with FF and APS-C both fitted with 50mm lenses at f8 (or any other combination of focal lengths and apertures) we end up with two pictures which look quite different if printed at any size we can imagine. If we then move back and increase your camera to subject distance the APS-C picture will still look different to the FF picture. This is why I personally think the crop factor is helpful as it's an easy way of equating something with something else. In my case equating APS-C or MFT to something I'm more used to, FF.

Maybe people who've grown up with APS-C don't need to know crop factors but they may then have a few confused moments if they pick up a FF or MFT camera.

Of course we will, because perspective depends on the camera to subject distance. That's nothing to do with depth of field.

 

[The Duke of Bork]

But instead of putting up and making do or spending time and money buying new lenses or bodies or both wouldn't it be better if we understood a bit more and were thus able to make better buying decisions?

But complaining that lenses are described by their focal length is so much howling at the moon. It's a fundamental physical property. The field of view varies according to the size of your sensor/film for a given focal length. There's no way around this.

 

[BADGER.BRAD]

Of course we will, because perspective depends on the camera to subject distance. That's nothing to do with depth of field.

But depth of field will be different between different sensor sizes, in the case of truely large format photography 10X8 it can become quite difficult because you may have the end someones nose in focus but not their eyes, with a much smaller sensor such as on most compacts or 110 film depth of field is much deeper to the point you couldn't blur the back ground if you wanted to.

Or learn to use what you have.

This only works in some circumstances if you need to stand back another 10 foot beyond the edge of the cliff in order to get the subject in frame or into the under growth which will now block the subject then one will be deadly and the other pointless !

 

[The Duke of Bork]

But depth of field will be different between different sensor sizes, in the case of truely large format photography 10X8 it can become quite difficult because you may have the end someones nose in focus but not their eyes, with a much smaller sensor such as on most compacts or 110 film depth of field is much deeper to the point you couldn't blur the back ground if you wanted to.



This only works in some circumstances if you need to stand back another 10 foot beyond the edge of the cliff in order to get the subject in frame or into the under growth which will now block the subject then one will be deadly and the other pointless !

But these limitations will apply whatever lens you're using. If you can't make the picture that's in your head with the equipment to hand, you have to take a different picture.

Or learn to paint.

 

[BADGER.BRAD]

But complaining that lenses are described by their focal length is so much howling at the moon. It's a fundamental physical property. The field of view varies according to the size of your sensor/film for a given focal length. There's no way around this

If you are buying fixed lens cameras then what is described as wide angle on one format will be totally different on another the difference in angle from a 110 camera with a 24mm lens to a large format camera with a 24mm lens would be amazing

 

[BADGER.BRAD]

But these limitations will apply whatever lens you're using

So your saying if I have to stand back at 50 feet with a 22mm lens using 135 film to get the building in frame then using a 200mm lens will be just as usable, it makes you wonder why people buy different lenses if they have no affect !

 

[Petrochemist]

Hello all,

I'm constantly messing with cameras with different size sensors and film formats and this Focal length thing is a total pain in the backside. I generally like a really wide angle lens as most of my photos seem to use that end of the lens. But trying to work out how wide a given lens is on a camera is a total pain surely there's a better way !

Nope.
Focal length is an intrinsic property of the lens & can readily be combined with the cameras crop factor to show how wide it will look (Provided the lens 's image covers the whole sensor).
If instead you refer to the field of view the lens gives this will change every time the lens is used on a different format, and it's not so easy to calculate.
I enjoy adapting lenses, and can use any of my M42 lenses on any of my (interchangeable lens) digital cameras this currently involves six different sensor sizes when using the camera's full sensor. The mental maths needed to estimate the expected FOV is simple enough that I've never needed a calculator. I think calculating it more accurately would only be of any use if I've carefully measured the FOV required & I don't carry my sextant routinely.

Things do get more complicated when the lens doesn't cover the sensor. My 6mm c-mount lens has much less FOV than my rectilinear 10mm FF lens, unless I've adapted them to a camera with a tiny sensor (with a crop factor ~5). Things also get complicated by fisheye lenses where focal length has little relation to FOV.

If considering film cameras too this currently adds something like another 6 sizes for my cameras (mainly medium format 645, 6x6, 6x7, 6x9, 6x12 but also 5"x4") M42 lenses will not cover those reliably (one or two lenses might cover 645 with luck). The same crop factor calculations can be used but coverage and he wildly varying image ratios are likely to be a much bigger issue. I tend to just get a feel for what lens A might look like on body B (if mounting it is practical) In time I might use the LF camera as an optical bench to determine that maximum coverage of my MF & LF lenses - just look out for the flying pigs!

 

[StephenM]

But depth of field will be different between different sensor sizes, in the case of truely large format photography 10X8 it can become quite difficult because you may have the end someones nose in focus but not their eyes, with a much smaller sensor such as on most compacts or 110 film depth of field is much deeper to the point you couldn't blur the back ground if you wanted to.

If you use a 10x8 camera with an appropriate focal length for portraiture at a suitable distance, we're talking about using a 360mm lens at an 8 to 10 feet distance. I strongly suspect that you'd find depth of field limited even on a 35mm camera with this lens and distance.

For a given subject distance, the longer the focal length the shorter the depth of field. That is why DOF is less for the larger formats - you need much longer focal lengths.

 

[StephenM]

@woof woof I didn't twig the effective aperture would also change, but obvious now it's pointed out as less of the image falls on the sensor.

EDIT: so for depth of field calcs I need to use the effective focal length, not the actual focal length?

All that happens when you move a lens from a full frame camera to one with a smaller sensor is that because the sensor is smaller it records less of the image from the lens. That's all. If you enlarge the same to the same degree (giving a smaller print from a smaller sensor) then depth of field remains the same. The aperture is a function of the size of the opening in the lens and the focal length, and has nothing to do with the camera the lens is attached to.

The reason depth of field can change is down to the degree of enlargement needed to get the same size print from a smaller format. Only one plane at a time can actually be in sharp focus; you get a falling off as the distance changes. Depth of field calculations depend on having a "standard" for what is acceptably sharp, or putting it another way, at what point the out of focus image of a point in the subject is no longer perceived as a point in the image. This depends on the degree of enlargement (twice the enlargement, twice the size of the small dot that a point is rendered at, and eventually is no longer seen as a point) and the viewing distance (things get bigger as your eye approaches them).

In @BADGER.BRAD's example, we're looking at the depth of field produced by a 360mm lens, at a subject distance of (say) 8 feet, with an 8x enlargement to compare with a contact print from a 10x8 camera. The fact that the 35mm camera would only record a nose is neither here nor there for our purposes, as the 35mm frame is just cropping the image.

Edit to add:
Focal length is simple; the complication comes from the unnecessary idea of a crop factor.

 

[Coho-Blue]

But instead of putting up and making do or spending time and money buying new lenses or bodies or both wouldn't it be better if we understood a bit more and were thus able to make better buying decisions?

Hope you All don`t mind me jumping in on this as its always something that I found to be confusing to a Degree ..

Yes it would be good to understand the whole DoF/field of view issue completely, however, I think a lot of hobby shooters ( including myself ) would benefit from a more user friendly way of understanding it as they grow into their photography journey and all the complications that comes with it ...

Would it not be better/possible to have a kind of comparison chart ? somethig that gives a rough guide of each lens on a FF Camera along with its Equivalent for APS-C

Lets say for Example I have a FF Camera and a ASP-C ,, If I buy a 50mm Lens for the FF Camera - what would be an Equivalent ??mm lens to buy for the APS-C in order to achieve a similar look ?

hope this makes sense


Coho - Blue

 

[woof woof]

Hope you All don`t mind me jumping in on this as its always something that I found to be confusing to a Degree ..

Yes it would be good to understand the whole DoF/field of view issue completely, however, I think a lot of hobby shooters ( including myself ) would benefit from a more user friendly way of understanding it as they grow into their photography journey and all the complications that comes with it ...

Would it not be better/possible to have a kind of comparison chart ? somethig that gives a rough guide of each lens on a FF Camera along with its Equivalent for APS-C

Lets say for Example I have a FF Camera and a ASP-C ,, If I buy a 50mm Lens for the FF Camera - what would be an Equivalent ??mm lens to buy for the APS-C in order to achieve a similar look ?

hope this makes sense


Coho - Blue

It's sort of easy...

Canon APS-C is x1.6 crop but I think everyone else who does APS-C is x1.5 crop. Micro Four Thirds is x2.

So if you have a FF lens and want to buy an equiv for APS-C it's focal length/crop factor so for Canon it's 50/1.6=31.25mm so really it's a 30mm or 35mm as I doubt you'll find a 31.25mm For the other APS-C makes it's 50/1.5= 33.33mm. MTF is much easier, 50/2=25mm.

I hope that helps.

 

[Crotal Bell]

As a beginner, the various complexities of modern cameras is quite daunting, even more so when more experienced people are posting there struggles with certain areas.

I'm already starting to throw the towel in and just "point what I've got and take what comes out"

 

[StephenM]

Depth of field depends ONLY on

focal length (longer = less depth of field at a fixed subject distance)

subject distance (the greater the distance, the greater the depth of field)

aperture (the smaller the aperture, the greater the depth of field)

degree of enlargement (out of focus areas show that they're out of focus as you enlarge them more)

viewing distance The closer you get to the print, the easier it is to see the fuzzyness. Think of approaching a "sharp" image on the billboard and see the sharpness vanish.

the size at which the circle in the image produced by a lens of a point no longer looks like a point (the circle of confusion)

That's it.

There can be an apparent contradiction to this in that depth of field tables can vary depending on the format, but this comes from adjusting the value of the circle of confusion to reflect the smaller degrees of enlargement from larger formats.

Format size controls how much of the image produced by a lens will be recorded. At infinity, the size of the image produced by a lens is directly proportional to the focal length. So as the format size decreases, so must the focal length of the lens used if you want to include the same amount. Hence why I find just working from the "standard" for each format really easy.

 

[The Duke of Bork]

So your saying if I have to stand back at 50 feet with a 22mm lens using 135 film to get the building in frame then using a 200mm lens will be just as usable, it makes you wonder why people buy different lenses if they have no affect !

No, I'm saying that if you have a 22mm lens, you'll find yourself wishing you had a 17mm. Then, when you get the 17mm, you'll find yourself in a situation where only a 12mm will get you the image you have in mind. And so on...

If you only have one lens, then you have to use that, even if it means looking for a different image.

 

[Petrochemist]

It's sort of easy...

Canon APS-C is x1.6 crop but I think everyone else who does APS-C is x1.5 crop. Micro Four Thirds is x2.

So if you have a FF lens and want to buy an equiv for APS-C it's focal length/crop factor so for Canon it's 50/1.6=31.25mm so really it's a 30mm or 35mm as I doubt you'll find a 31.25mm For the other APS-C makes it's 50/1.5= 33.33mm. MTF is much easier, 50/2=25mm.

I hope that helps.

Sigma did a 1.7 crop APSC (I don't think they do now)
Then there's the old Nikon 1 (cx), Pentax Q series, NX... quite a few other crops available if you use older cameras but still simple enough to know what you use.