Focal lengths clarified

[mick1985]

Guys,

Sorry for what may very well be a totally silly question, I had a search and could not find the info I was looking for, but im currently trying to get my head round focal lengths and their affects on distances.

As I understand it, using a long focal length lens (for example a "bigma" at 500mm, will allow me to photgraph a subject at a further distance than say a 70-200 f2.8 at 200mm, whilst still having it fill the picture, without the neex to crop down later (hence getting a clearer, more detailed picture?)

I also understand my camera has a "crop" sensor (d3100 and a 1.6x crop iirc?) meaning a 500mm lens on my camera is the equivalent of a 800mm lens on a FF sensor camera, all correct so far?

But what does the 500mm represent? Is that the physical length of the lens? The length of the sensor to the end of the lens?

Finally, is there a calculation or a ready reckoner table to work out ho much "zoom" a given focal length gives you? (does a 500mm lens make ten feet seem like afoot away etc?)

As I say, probably fairly rudamentary questions, but I have no real experience of long zoom lenses, am thinking of getting one, but would like to understand it all a bit more.

Thanks in advance,

Mick

 

[DazJW]

Focal length a physical measurement to do with what the optics of the lens do to the light they're focusing.

Are you intimately familiar with how focal lengths look on a 35mm film or full frame digital camera or are you planning to run different formats at the same time? If not then forget about anything relating to crop versus full frame as it's irrelevant to your photography. Don't waste your time thinking "Well I might buy a full frame camera in a few years time so I need to know" because you probably won't buy one and even if you do you can worry about it then. You'll know more about photography and cameras by then so it'll be much easier to understand the effects of the different formats.
The short version is that you camera physically crops the image the lens projects more than a larger format would. It doesn't affect focal length, only field of view.

You could use a magnification calculator to work out how much a lens will magnify an object at a given distance. You'll need to find the right circle of confusion for you sensor format.

 

[dinners]

500mm on a crop will 'appear' like 800mm on a full frame but all that's happening is your crop camera sensor is cropping the peripheral part of the image making it appear to be nearer.

As for what does 500mm focal length actually mean - I'll swipe rather try to explain in my own words "distance in mm from the optical center of the lens to the focal point, which is located on the sensor or film - if the subject (at infinity) is "in focus".

 

[mick1985]

Hi Darren, thanks for the reply.

Nope, never used a film camera in my life, and in all honesty probably never will. This is not so much a cropvsfull frame question, as I will eventually move over to FF when I see fit. This will not however, desuade me from buying lenses for my crop camera for now.

I was simply asking as I would like to gain a fuller understanding of how the mechanics of my camera, and its optics functions. As being a mechanical engineer in my work life, things of this nature interest me, and I find that for me myself, having a full understanding of how things work, allows me to manipulate them better, ad I understand whats happening when I adjust focal length etc.

The calculator you linked to seems interesting, will need to have a play with thag later. Just read the wiki article also, I understand about what the 500mm I used as an exampe means now, so thanks for that.

I assume in a zoom lens there are three or more lenses that slide inside the casing which is how the focal length is altered?

Cheers

Mick

 

[mick1985]

500mm on a crop will 'appear' like 800mm on a full frame but all that's happening is your crop camera sensor is cropping the peripheral part of the image making it appear to be nearer.

As for what does 500mm focal length actually mean - I'll swipe rather try to explain in my own words "distance in mm from the optical center of the lens to the focal point, which is located on the sensor or film - if the subject (at infinity) is "in focus".

Ah, that makes sense. So basically the focal length is still the same (as it is a mechanically set value) but how the sensor percieves itis different (i.e no more or less zoom than a ff would have, just a tighter crop straight out the camera?)

 

[aleung2]

That's help me understand it too. I think what makes it more difficult to understand as a beginner is that field of view and focal length. Now I get it!!

 

[DazJW]

As being a mechanical engineer in my work life, things of this nature interest me

You'll probably be alright with it then. The nature of my reply was because a lot of beginners get really bogged down in trying to understand it when they simply don't need to.
Essentially it's like taking a TV screen and putting duct tape around the outside edge until the section you can see is only half as big as the whole screen, the image looks more zoomed in but it isn't really, there's just less visible. Having sensors that produce different image resolutions complicates this a bit to the point it doesn't really matter.
The focal length multiplier thing was to help people who were used to using 35mm cameras to visualise how the same focal lengths would look with the reduced field of view from a digital camera which had a sensor smaller than the 35mm film frame. Optically the focal length is Xmm regardless of what format the lens is mounted to.

I assume in a zoom lens there are three or more lenses that slide inside the casing which is how the focal length is altered?

The Wikipedia page for zoom lenses has a section on design which includes a simplified animation of the internals of a zoom lens and some further explanation.

 

[dinners]

Ah, that makes sense. So basically the focal length is still the same (as it is a mechanically set value) but how the sensor percieves itis different (i.e no more or less zoom than a ff would have, just a tighter crop straight out the camera?)

Yes.

A 500mm lens is a 500mm lens regardless of whether you have it on a 1.6 crop sensor camera or a full frame sensor camera.

When it's on a full frame camera you see the whole image but when it's on a crop camera you loose the edges making it appear more zoomed in.

As per your maths - On a full frame camera you would need to use a 800mm lens to see the same area as you would through the viewfinder of a crop camera with a 500mm lens attached.

Generally speaking that's why you see lenses like a 24-70mm or 17-40mm designed with full frame in mind and lenses like 17-55mm or 10-20mm with crop in mind.

 

[mangust]

Look at it as definition of angle of view. Not zoom.
more zoom mean less angle of view.
focal length is somewhat if you have just simple lens of single glass, not group of them as we have in modern lens. Then you need to put it 500mm apart of paper/sensor to get a picture. If you use group of lenses you can make physical size of lens smaller. So that modern 500mm lenses are significantly smaller then 0.5 meter.
Consider lens as not magnifier but something that make image smaller. In most cases it is. The closer lens to the sensor, smaller image on the sensor. Just like home cinema projector. If you want to magnify such cinema then you need to place screen on more distant wall. If instead of wall size screen will be used A4 paper (smaller cropped sensor). Then content around that paper will be cropped away and you'll get extra magnified fragment of original wall size picture.

 

[mick1985]

Thats great fellas thanks, its pretty clear in my head now.

Plenty of info in the wiki page that was linked, the tv duct tape analogy is also a goodun.

Just need to think about getting some good pics now LOL!

Thanks again, its well appreciated

 

[brokenlynx]

Thats great fellas thanks, its pretty clear in my head now.

Plenty of info in the wiki page that was linked, the tv duct tape analogy is also a goodun.

Just need to think about getting some good pics now LOL!

Thanks again, its well appreciated

I'd also suggest this site to help with the idea of focal length and crop. Basically a beginners guide to photography for those who are a bit more 'au fait' with maths:

http://berniesumption.com/photography/beginners-guide-for-geeks/

It's a good guide and should fill in some gaps for you.

 

[redhed17]

Just to point it out, because I don't think this has been pointed out so far, but the D3100 (and all Nikon DX/APS-C cameras) have a 1.5x crop factor, as has the other APS-C sensor cameras from every other manufacturer apart from Canon, whose APS-C cameras have a slightly smaller sensor, and have a 1.6x crop factor.

So the 500mm lens on a D3100 would have a crop factor of 750mm.

Four Thirds, and Micro Four Thirds cameras have smaller sensors, and have a crop factor of 2x. So that 500mm lens would have crop factor of 1000mm.

The image here shows the sensor sizes relative to each other. And this image shows how the crop of the smaller sensor, and how the recorded scene with appears to have been zoomed in closer.

 

[StewartR]

Finally, is there a calculation or a ready reckoner table to work out ho much "zoom" a given focal length gives you? (does a 500mm lens make ten feet seem like afoot away etc?)

I'm not sure whether this answers the question you had in mind, but for telephoto lenses you can make use of the following relationship:

Subject size / Subject distance = Sensor size / Focal length

This will enable you to visualize the effect of a lens, or determine what size lens you need.

For example, suppose you want to photograph a polar bear with a "Bigma" (500mm). The bear is say 3m long, and the long side of your sensor is 24mm, so the bear will fill the frame at a distance of 3 *500 / 24 = 62.5m. In practice you'd want to allow the subject some room to breathe in the frame, which means you'd be fine at up to about 80m or so.

There's a more accurate relationship involving trigonometry which would also work for wide-angle lenses and close subjects, but 99% of the time it's telephoto lenses and distant objects which are of interest in calculations like this, so I've kept it simple.

 

[Roy C]

Just to throw another spanner in the works with regards to focal lengths, with a zoom lens like the Bigma even if you are at 500mm it will almost certainly be less than that in reality - the nearer you are the less it will be. I saw a test once between a 400mm prime and the 'Bigma' set at 500 and the actual focal length difference when shooting the same target from around 15 ft was surprisingly little. Several threads I have read on the subject suggest the Bigma is around 450mm at its longest when shooting at normal distances for birds (if there is such a thing lol). I think it is only when you get to infinity that most zooms are near to the reported max focal length.

 

[new2me]

Subject size / Subject distance = Sensor size / Focal length

This will enable you to visualize the effect of a lens, or determine what size lens you need.

Thanks for this nugget.
I was going to ask such a question a while ago, but didn't get round to it