Lenses technical questions

[E-Photographer]

Hi everybody,
I have some technical questions regarding lenses on a digital body (20D).
I know that at a close distance from the subject, a wide angle lens creates deformed prospectives.

1) A 28mm lens tends to affect the prospective in a significant way. When I use a 28mm on a 20D, I get the equivalent of a 44.8mm on a full-frame sensors due to the POV being 1.6. Will the final image have a deformed prospective equivalent to a 28mm lens (so quite severe) or a 44.8mm (quite moderate)?

2) If I use a 200mm on a 35mm SLR, I get a nicely blurred background (at big apertures). On a 20D it becomes the equivalent of a 320mm. Does this affect the depth of field or do I get the same effect as with film? In other words, do I still get a blur like a 200mm or do I get a blur like a 320mm? I suspect that depth of field is only influenced by real focal length, but could you clarify this?

3) I know it is possible to use a telephoto inverted and attached to a ring adapter, to use it as a macro lens. But an inverted telephoto doesn't become a wide angle lens that is good for landscapes. Why?
Why does inverting a telephoto allow the minimal focus distance to drop, making the lens good for Macro work?

4) I am aware of the value of a Canon EF 17-40mm L f/4. On a 35mm or on a full-frame sensor, you spend £500 and you get a great wide-angle zoom, wonderful for landscapes and small spaces indoors. I honestly can't however understand the value of this lens on a 20D. It becomes the equivalent of a 27.2-64mm. Ok it is a walkaround lens, but I believe that the reason why that lens costs so much (besides build quality) is because it offers you a good wide angle, that on a digital 1.6x sensors, gets lost. Could you explain me why you think this lens is worth its money on a 20D? It looks very popular in the forum.

5) Using a 17-40 L in an indoor studio for full-body portraits with fashion models, doesn't generate a deformed prospective which would go against my models? Many fashion photographers use telephotos to avoid knees and hands from jumping into the front of the picture and looking weird. Is it a good lens for the purpose of capturing full-body/half-body lengths in a small studio?

6) For a business based on portraits and fashion, indoor and outdoor and some sport shots, what do you think about the following set of lenses on a 20D body: 17-40mm L f/4, 50mm f/1.8, 70-200mm L f/4?

Thank you for clarifying my doubts and I am sure your answers will also be helpful to other people that don't dare to ask :-)

[tim]

First off, unless you have years of 35mm experience, don't worry about it, look through the lens and accept what you get. These threads almost always turn into an argument that doesn't help anyone. I'll tell you what I think anyway.

1) Perspective isn't changed by the size of the sensor.
2) I suspect what you said, but the DOF calculator (http://www.dofmaster.com/dofjs.html) suggests otherwise.
3) No idea.
4) Not sure why, I suspect it has quite good image quality and it's still wider than most. People who want really wide go for 10 or 12mm lenses on a 1.6X camera.
5) I prefer to use greater than 50mm lenses for models if I can. My 100mm macro lens is great for this if I have space, but I usually don't for full body shots.
6) It depends on your style. I use my Tamron 28-75 a lot, and the 40mm probably wouldn't be long enough for me, but maybe if your studio's small it'd work. Forget the 50mm F1.8, it's cheap for a reason, though it does well for undemanding situations, stopped down and at a distance from the subject. The F4 zoom is a bit slow for low light sports, if you're planning on doing them, and a bit short for some sports. Again, it depends.

Hope that helps :)

[pietzcker]

Hi,

My take on questions 1) and 2):

Keep in mind that the picture is only cropped on a DSLR. It's as if you take a normal film negative and cut off the borders until the resulting image is 1.6 times smaller than before. From there it follows that the optical characteristics of the lenses remain the same, so distortion on a 28mm lens doesn't go away, nor does a 200mm lens change its DOF characteristics. As far as distortion is concerned, however, you will probably see an effect because distortion usually is most pronounced in the outer areas of the picture - and your DSLR sensor doesn't cover the outer areas. The wide-angle characteristics (e.g., large noses in close-ups) remain the same - but if you "fill the frame" on a full-frame sensor with a certain subject (say, a face), you are closer to the subject than if you fill the frame on a 20D with the same face - so those characteristics are somewhat moderated on the 20D. I hope I'm making sense to you :)

So if you want to judge how your lens's distortion will look like on a 20D, take a picture with an analog camera and cut off the sides and top until your picture is 1.6 times smaller. Then you'll see what it'll look like on the 20D.

HTH,
Tim

[accord]

I'll try to answer number 2).
Focal length and aperture are physical constants, they won't change with crop factor.
If you follow the DOF calculator mentioned by Tim, you will find that
a) Given a focal length f, DOF is inversely proportional to the square of f.
b) Given an aperture F/a (for f/1.4, a=1.4), DOF is proportional to a.
c) Given a shooting distance d, DOF is propotional to the square of d.

We will use the approximate result of a), b) ans c) for comparing the DOF of lens in a FF and a 1.6x crop system.

Example 1: What is the shooting distance for the same size of image if I use a 200mm lens?
It is simply a result of geometry, You'll need 1.6 times more distance for a lens to shoot the same size image for a 1.6 crop factor system.

Example 2: What conditions will provide the same DOF for a 200mm lens in a FF system for the same size of image?
i) The same 200mm lens is being used. The shooting distance d will needed to be increase by the crop factor (1.6 in this case), DOF will be increased by factor 2.56, c) above.
ii) The same distance but swap a lens, a 125mm lens will provide the same size of image. The DOF will be increased by 2.56, a) above.

Example 3: How much DOF control I lost when using a 1.6 crop system?
This question may be rephased as how much larger the aperture I need for the same DOF in a 1.6 crop system?
The answer is simply 1.83 (2.56/1.4) stops, b) above.

[Andy_T]

e-photographer,

you got quite some correct answers so far (and a few not completely correct ones, but that was to be expected :wink: )

1) perspective is only influenced by your distance from the subject.

Of course, if you fill the frame of a wide angle lens with your subject by moving very closely to it, then the perspective changes (because you move closer to it).

If you take a slide with a film camera at 28 mm from a big distance and cut the slide so that it has the same framing as on a shot with a 200 mm lens and then enlarge it more to get the same size, you get the same perspective. DOF will be different, though, because it also is affected by the 'circle of confusion' and the enlargement.

2) A bit technical to explain (and I can't do it ... you must search the forum), but yes, it does change (a bit). Very much visible if you use a small sensor P&S - they have a 5x crop factor, and it's nearly impossible to get a nicely blurred background with these. With the 70-200/4.0 on a 20D you will have very good possibilities to isolate your subjects from the background.

If you want to have more details, you have to try to understand Accords post :lol:

3) Don't know that, but my best guess is that a landscape lens that can not focus to infinity (like a macro lens) would be a bit unusable. Also, there will be a lot of vignetting.

4) Simple answer ... because 27 mm is definitely better than 44 mm, if you want to take landscapes :p
There was a time when 28 mm *was* considered wide angle. I still see it that way, although I also would like to have a true 20 or 24 mm on my 20D. (like the new super wide lenses).

5) see 1)

6) Highly subjective - depends on your shooting style. Fashion, indoor and outdoor doesn't exactly shout 'wide angle' to me, so I would rather replace the 17-40 with a Sigma 24-70/2.8 EX or Tamron 28-75/2.8 XR DI and get the 18-55 kit lens for the occasional wide angle shot (ok, I actually *did* go that route :wink: ). The 18-55 is quite decent on the wide end.

For professional fashion shots, I would suggest using the 50/1.4 instead of the 50/1.8 for the reasons outlined in this thread: 50 1.8 ugly bokeh (http://photography-on-the.net/forum/showthread.php?t=69785)

Best regards,
Andy