Originally posted by normhead Funny, I once shot a small flower with a ruler, a 50mm lens to represent the FF camera and a 35 on 16 a K-5, to represent the APS-c, at the same settings and used the same framing, and got twice as much DoF with the 35 from exactly the same tripod mount position and ƒ-stop.
First, apologies for using "narrower" when I meant "deeper". Since you didn't use the common "shallower" I thought you meant the opposite of "shallower", which is deeper. Sorry, for getting mixed up.
However, your example does not make sense.
- You cannot "represent the FF camera" by just sticking a longer lens on an APS-C camera. The sensor size must grow as well. You could have simulated a four-thirds camera (to compare it to APS-C), by cropping a four-thirds portion out of the APS-C frame.
- Changing the distance to achieve the same framing is not a basis for a good comparison. By changing the distance, you also change perspective, i.e., you create a different image with different foreground/background proportions, and different DOF properties. The proper way to achieve the same framing is by using different focal lengths (and different sensor sizes).
- Why don't you visit a DOF calculator some time and change the sensor size in addition to the focal length (the equivalent APS-C focal length is closer to 33mm than to 35mm, BTW) and verify the "claims" that jsherman999 and I are making?
Originally posted by normhead So, I'm not sure what you just said up there but it doesn't apply to my experience with real world shooting.
What I said -- which is much easier to read after I replaced "narrower" with "deeper" (which it always was intended to mean) -- is in accordance with physics, DOF calculators, and properly conducted experiments.
Originally posted by normhead And why would you do that?
I would never use the same lens on two different formats with the same settings, unless I wanted to produce two completely different images.
So I agree with you, the thought experiment does not have much merit. The only reason I did it, was to make sense of your statement "
Shooting FF means narrower depth of field" while misreading "narrower" as the opposite of "shallower".
Indeed, there is a situation when FF yields a deeper DOF, which is when you use the same lens on both formats and keep the same settings. That's the only reason as to why I brought this scenario up. My bad for not catching your true intent.
BTW, the statement "
Shooting FF means shallower depth of field" (which is what you meant) is not true either. You can always stop down an FF lens, so that you get the same DOF as with an APS-C camera. The only exception to this rule is when you run out of f-ratios, say at "f/22" or so. You cannot replicate the DOF of a 33mm f/22 shot on APS-C with a 50mm f/33 shot on FF, because there is no 50mm lens that can be stopped down to f/33. However, such high f-ratios are not useful anyhow, as they dramatically limit resolution due to diffraction. You don't need more than ~2MP to capture the resolution of an f/22 image on APS-C. Such images are typically not worth taking, AFAIC.
Originally posted by normhead You go for the shot you want with the equipment you have, and you pick the right lens for that format.
Sure, I 100% agree.
However, if you want to know whether it is worth spending money on FF equipment, it is good to have ways to make proper (fair) comparisons. That's when equivalent settings come in handy, otherwise you are just comparing apples to oranges.
Originally posted by normhead It sounds an awful lot like..." if you don't agree with what I say , I can think of some stupid scenario where I'm right and you're wrong."
With all due respect, it was you who used that tactic when you grabbed "ISO 1600" out of thin air for the FF camera and then argued that high ISO values decrease dynamic range, etc. Clearly, the correct value to pick was "ISO 225" and picking anything higher only serves to drive home a point (for which no proper argumentation exists).
I never manufactured any examples to make APS-C look bad. I only state facts supported by physics and practice.