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12-18-2008, 08:31 AM   #1
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Lens markings

I'm curious. If I focus on something, and I look at the lens, and it tells me that I'm 10' away from the subject. Is what the lens showing me accurate, or not because of the crop factor?

12-18-2008, 08:41 AM   #2
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QuoteOriginally posted by eccs19 Quote
I'm curious. If I focus on something, and I look at the lens, and it tells me that I'm 10' away from the subject. Is what the lens showing me accurate, or not because of the crop factor?

Accurate.

The math does not involve the sensor size. Imagine an image in focus on a big sensor; it is also in focus on a smaller part of that sensor.
12-18-2008, 08:41 AM   #3
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although the hyperfocal markings are off by about a stop
12-18-2008, 08:42 AM   #4
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the crop factor desn't change distance between you and the object, it just changes FOV for specified focal length. so if lens is telling you that you are 10 feet away, you are 10 feet away. The difference is, if you put the same lens on FF/film camera from the same 10 feet looking at the same subject you will see more in you VF...
BR

12-18-2008, 08:46 AM   #5
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QuoteOriginally posted by k100d Quote
although the hyperfocal markings are off by about a stop
please correct me if I'm wrong:
APS-C sensor sees less than full frame designed lens. That means if you focus HF on let's say f8 you are actualy wasting some space in focus (because the smaller sensor doesn't see it) and therefore you could use smaller f-stop and (f5,6 and still get into focus roughly what you need/want?
Do I understand this correctly?
12-18-2008, 09:12 AM   #6
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QuoteOriginally posted by axl Quote
please correct me if I'm wrong:
APS-C sensor sees less than full frame designed lens. That means if you focus HF on let's say f8 you are actualy wasting some space in focus (because the smaller sensor doesn't see it) and therefore you could use smaller f-stop and (f5,6 and still get into focus roughly what you need/want?
Do I understand this correctly?
You're talking about depth of field, right? The difference in depth of field isn't anything to do with the lens, it's because you have to enlarge the image from the smaller sensor MORE to get a print or display image of the same size. This means that the slight blurring of out-of-focus areas is magnified, and so the range of objects with "acceptable blurriness" is reduced. This is comparing the SAME LENS on different sensors (which of course gives different fields of view).

What you're saying is the opposite of what it should be - f/5.6 is a "larger" f/stop than f/8 (because the iris has a larger opening at f/5.6 than at f/8). And FOR THE SAME LENS, you need to use a smaller f/stop on APS-C (ie, use f/11 vs. f/8 on full frame) to get similar depth of field.

Smaller sensors have larger depths of field at a given aperture if the FIELD OF VIEW is the same, which means using a shorter lens with the smaller sensor.
12-18-2008, 12:42 PM   #7
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Thanks for the info. I was wondering about this, because my flash lists distance on the back, and recommended F: setting, so I was hoping it was accurate. Helps a lot.
12-18-2008, 01:22 PM   #8
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It should be accurate, just run out a ten foot tape measure and see, it should be there or there abouts.

12-18-2008, 02:22 PM   #9
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QuoteOriginally posted by kerrowdown Quote
It should be accurate, just run out a ten foot tape measure and see, it should be there or there abouts.
Anyone should be able to eyeball the difference between ten and fifteen feet.
12-18-2008, 06:55 PM   #10
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QuoteOriginally posted by axl Quote
please correct me if I'm wrong:
APS-C sensor sees less than full frame designed lens. That means if you focus HF on let's say f8 you are actualy wasting some space in focus (because the smaller sensor doesn't see it) and therefore you could use smaller f-stop and (f5,6 and still get into focus roughly what you need/want?
Do I understand this correctly?
QuoteOriginally posted by Sean Nelson Quote
You're talking about depth of field, right? The difference in depth of field isn't anything to do with the lens, it's because you have to enlarge the image from the smaller sensor MORE to get a print or display image of the same size. This means that the slight blurring of out-of-focus areas is magnified, and so the range of objects with "acceptable blurriness" is reduced. This is comparing the SAME LENS on different sensors (which of course gives different fields of view).

What you're saying is the opposite of what it should be - f/5.6 is a "larger" f/stop than f/8 (because the iris has a larger opening at f/5.6 than at f/8). And FOR THE SAME LENS, you need to use a smaller f/stop on APS-C (ie, use f/11 vs. f/8 on full frame) to get similar depth of field.

Smaller sensors have larger depths of field at a given aperture if the FIELD OF VIEW is the same, which means using a shorter lens with the smaller sensor.
sounds right to me
12-19-2008, 12:56 AM   #11
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thanx for clarifying that for me guys, seems there is still a lot to learn...
anyway, since the APS-C doesn't see as much as FF (since the FOV of the same lens on two different formats is different) you may not need the same DOF, or not?
so now if I'm using my Tokina 28 (old manual lens from film era) shall I be stopping it down stop more to get everything in focus while set on HF?
Sorry, I'm just getting really confused here...
12-19-2008, 01:35 AM   #12
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QuoteOriginally posted by axl Quote
thanx for clarifying that for me guys, seems there is still a lot to learn...
anyway, since the APS-C doesn't see as much as FF (since the FOV of the same lens on two different formats is different) you may not need the same DOF, or not?
so now if I'm using my Tokina 28 (old manual lens from film era) shall I be stopping it down stop more to get everything in focus while set on HF?
Sorry, I'm just getting really confused here...
For a given focal length, aperture and distance, you'll get more DOF when used on a cropped sensor than on FF.
12-19-2008, 02:16 AM   #13
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QuoteOriginally posted by creampuff Quote
For a given focal length, aperture and distance, you'll get more DOF when used on a cropped sensor than on FF.
You see, this is why poor Axl is getting confused. Your statement is incorrect, it should be "for a given FIELD OF VIEW", not for a given focal length.

The following calculations are from the DOF calculator at: Online Depth of Field Calculator using an aperture of f/4 and a subject distance of 2m:

Same focal length lens gives LESS depth of field on a smaller sensor:
50mm lens on 35mm film has a DOF of 0.38m
50mm lens on K10D has a DOF of 0.25m

Same field of view gives MORE depth of field on a smaller sensor
50mm lens on 35mm film has a DOF of 0.38m
33mm lens on K10D has a DOF of 0.59m
12-19-2008, 02:25 AM   #14
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QuoteQuote:
Axl said:
... using my Tokina 28 (old manual lens from film era) shall I be stopping it down stop more ... "
Yes, if you refer to the DOF scale on your Tokina lens.

This seems illogical, but it is true nonetheless. On old manual 'full-format' lens scales (=the DOF markings on the lens barrrel), if you use this lens on an APS-C camera, your DOF will shrink about 1.3 stops from what the scale tells you.

Example here: Digital Depth of Field
(scroll down til the end of the article).

So yes, if you use that old manual lens on APS-C and stop down to f/8, your DOF will shrink to something within the 5.6 markings on that lens barrrel.

Trust me :-)
Georg (the other)
12-19-2008, 02:34 AM   #15
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QuoteOriginally posted by axl Quote
so now if I'm using my Tokina 28 (old manual lens from film era) shall I be stopping it down stop more to get everything in focus while set on HF?
As a rule of thumb, I'd stop it down one extra stop (or conversely, use the DOF markings on the lens for the aperture one stop wider than what the lens is actually set to). So if the aperture ring is set to f/8, for example, use the DOF scale markings shown for f/5.6. You can put some figures into the DOF calculator I linked to in my previous post and see that the result is very close to the DOF you'll get from the smaller sensor.

And to repeat for those aren't getting it: the reason the depth of field is smaller is because the DOF scale on the lens is designed for full-frame 35mm film. When you use the same lens on a K10D, you have to enlarge the image MORE in order to get a final print or screen image of the same size (say, 10 inches across). Because you've enlarged the image more, you've also enlarged the blurriness of the slightly out-of-focus areas - that means some of those areas that would have appeared sharp before are now revealed under the extra magnification as not-so-sharp in the final picture. The net effect is: smaller depth of field.

(Of course the prints from 35mm vs. K10D, while the same physical size at, say, 10" across, wouldn't show the same picture because the field of view on the K10D is much smaller. But that's irrelevant to the depth-of-field issue at hand).

Keep in mind that DOF is somewhat subjective and highly dependent on the size and viewing distance of the final image. It's not an absolute thing. You'll have more apparent DOF in a small print or screen image than a large one, and more DOF in a print or image viewed from farther away than from one viewed at a closer distance.
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