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12-12-2008, 03:29 AM   #1
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Am I confused about DOF and full frame lenses?

Given:
The DOF is determined by the physical size of the lens aperture.
So,
the lens of a medium format camera requires narrower apertures (like f/22) for maximum DOF (while a P&S may require f/5.6, for example).

And,
1) For a certain aperture value (say f/2.8), full frame lenses (like those of the FA series) provide less DOF in comparison to APSC sized lenses (like those of the DA series) since their apetures are pysically larger.
2) For a fixed DOF (say, 10cm) a narrower aperture has to be used on a full frame lens as compared to an APSC one, to get a similar physical size (similar to the P&S example above).

Does this follow?:
You have two lenses (one FA & one DA) with the maximum aperture value of f/2.8
and
the required DOF is provided by an aperture value of f/2.8 of on the APSC one;
you will have better performance (resolution, sharpness, contrast etc)
by using the full frame one at (for example) f/4 since you are not using it at its extreme (since lenses perform relatively poorly at their extremes)
.

My (perhaps confused) logic outlined above also implies that for a fixed aperture value you can shoot at faster speeds with a full frame lens as more light will be reaching the sensor. (Since the distribution of light to a larger image circle is not uniform, the central part -occupied by the smaller sensor- will receive comparatively more light). Isn't that the reason we have inconsistent exposure readings using non-DA lenses?

Could someone please tell me if I am mistaken before telling :
"Man, go shoot and forget about these!".

Thanks,

ADDED: For the same reason, comparison of two lenses (hypothetically made of same quality glass and coated by the same chemicals) at their (say) f/2.8 on an APSC camera will favor the DA variety since its aperture will be physically narrower. This may explain the relatively less shiny comments on the large-aperture FA lenses you may see here and there.


Last edited by bc_the_path; 12-12-2008 at 03:55 AM. Reason: Added more...
12-12-2008, 05:19 AM   #2
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I think your assumptions may be wrong. DOF is defined by two things: appeture and magnification. In turn, magnification is defined by focal length and subject distance.

(This means that for a given subject size, the DOF does not vary with focal length, which is important for macro work - 1:1 has the same DOF at any focal length, assuming same sensor size)

In turn appeture is not impacted by FF/APSC - it is the ratio of lens width to focal length. A 50mm f2.8 has the same appeture regardless of FF or APSC.

Make sense?
12-12-2008, 06:30 AM   #3
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Merhaba Bülent,
you're lucky because I can reduce your question :-)

I know there are a lot of threads about DOF here and some people are already tired of it, but never mind, here goes:

DOF of one given lens is exclusively (no other factors) determined by:

- FL focal length (independent of sensor size, FL is always the same)
- A aperture area divided by focal length (independent of sensor size, aperture = FL / aperture diameter), just like jfsavage pointed out already
- COC circle of confusion: the degree of acceptable sharpness/unsharpness (dependent on sensor size, full format COC ~ 0,03mm, APS-C COC ~ 0,02mm) This is actually the easiest way to describe magnification ratio and viewing distance in one simple number.
- distance to subject (obviously independent of sensor size)

So I have reduced your question to four factors. Please believe me, nothing else contributes to the DOF.

Common myths:
- special 'digital' 'reduced-sensor' lenses with different optics than 'fullformat' or 'film' lenses (the industry was quite successful with this myth, since so many people believe they have to buy 'digital' lenses)
- exposure differences due to that special lens design (it is only that lenses with a chip inside can tell the camera much better what is going on. Also, some non-chipped lenses use the contacts of the camera bayonet better/different than others)

I think the easiest to start with is the basic lens design, like focal length and aperture, and what is actually inside a lens.
That is quite simple after all.

Hope this was not too much of things that you already know,
Georg (the other)

PS: Nice article on Digital Depth of Field (start at the end with points 1. - 5. )


.

Last edited by georgweb; 12-12-2008 at 06:39 AM.
12-12-2008, 11:26 AM   #4
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the confusion arises when you compare formats, most importantly HOW you compare them.

if you stand in the same spot and photograph something 20 feet away, with a DA and an FA 50mm lens at f2.8, you will get the same physical depth of field


but people dont do that, they compare field of view, and for that, using a DA lens, you have to take a few steps back so that your field of views are the same

and as you step back, you get a larger in focus corridor, hence the difference in depth of field.

12-12-2008, 12:06 PM   #5
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QuoteOriginally posted by Gooshin Quote
if you stand in the same spot and photograph something 20 feet away, with a DA and an FA 50mm lens at f2.8, you will get the same physical depth of field
Gooshin, if you don't mind, I'd like to clarify this statement for the benefit of the OP: In this example the DA lens is used on an APS-C camera while the FA lens is used on a full-frame camera.
12-12-2008, 12:18 PM   #6
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QuoteOriginally posted by Miserere Quote
Gooshin, if you don't mind, I'd like to clarify this statement for the benefit of the OP: In this example the DA lens is used on an APS-C camera while the FA lens is used on a full-frame camera.

well, it doesnt really matter what the format or lens is, i was just trying to illustrate that focal length is focal length,

if you have a FF and an APC camera, and you have a DA50 and an FA50, you can interchange all you want (4 possible combinations) if you stand at one spot and photograph a static object 20 feet away, DOF will not change.

the field of view will change, most definetly, but the physical focus corridor wont.
12-12-2008, 12:45 PM   #7
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Well, I know nothing of math or formulas, but when I added a 67II to my kit almost 10 years ago, the "halving" of the depth of field was glaringly obvious. I phoned one of the techs at Pentax Service Center and he told me that it was similar to the fact that a 200mm lens on 67 is considered equivilent to a 100 with 35mm. Field of view is halved and depth of field is similarly halved. I didn't press him for additional details. I was searching mostly to explain the noticeable phenomenon I saw in my images.
To this day I don't know how to convert my 35mm Hyperfocal Distance charts to work with the much larger 6x7 format.
So by that same token, then do our "mini" sensor cameras with much smaller image area than 35mm also increase the depth of field? Based on what I see coming from my digi bodies and others posted here, it appears that the shallow dof settings still get way more acceptable focus depth than did 35mm and ridiculously more than 67 format (or is that a software effect with sharpening?). Basically, my f. 4 medium format lenses have the depth of field of a 2.8 lens for 35mm and seem closer to about f. 2 on the digi bodies. I shoot primarily FA* lenses on my digital sensor bodies. If the lens alone defines the dof, shouldn't digi and 35mm be the same if shot through the same lens? And if I try a 67 to K mount converter to use the medium format lenses on a 35mm and also on a digi body, will I see the dof of the lens or the dof of the image size?
Partly I'm baiting the formula guys as there is no question that medium format lenses have half the dof of 35mm. I've seen it in many images and confirmed it with Pentax many years ago. The real question is...do our digi bodies make the dof of 2.8 (from 35mm days) when shot at f. 2? And does the lens itself even matter? Note that my first hand experience with comparing 67 and 35mm was not through the same lenses but rather by looking at results from each size kit compared to the other size. Anybody do any actual tests--comparing dof from the same lens on 35mm film and on digi sensor? Just curious...
12-12-2008, 12:53 PM   #8
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Also, are there hyperfocal distance charts available for APS-C cameras/lenses?

12-12-2008, 12:55 PM   #9
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QuoteOriginally posted by Ron Boggs Quote
...there is no question that medium format lenses have half the dof of 35mm.
It's not the lens, Ron, it's the medium you're shooting on. As somebody stated further up, if you shoot the same subject with different sensor sizes such that it appears the same size in the final image, independently of what lens you use on any of the cameras, the DoF will be determined only by the aperture you shot at. If you shot all lenses at the same aperture, the image from the APS-C camera will have a DoF about 1.5x deeper than the full-frame camera will produce. Likewise, the full-frame image will have a DoF that is about 2x deeper than the image from the 6x7 camera.
12-12-2008, 12:57 PM   #10
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QuoteOriginally posted by Ron Boggs Quote
Also, are there hyperfocal distance charts available for APS-C cameras/lenses?
If you're willing to install a little program on your computer, you can use this. It's pretty neat, and you can print charts for all your different film/sensor sizes.
12-12-2008, 01:01 PM   #11
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QuoteOriginally posted by Ron Boggs Quote
Partly I'm baiting the formula guys as there is no question that medium format lenses have half the dof of 35mm. I've seen it in many images and confirmed it with Pentax many years ago. The real question is...do our digi bodies make the dof of 2.8 (from 35mm days) when shot at f. 2? And does the lens itself even matter? Note that my first hand experience with comparing 67 and 35mm was not through the same lenses but rather by looking at results from each size kit compared to the other size. Anybody do any actual tests--comparing dof from the same lens on 35mm film and on digi sensor? Just curious...
thats because you are going to walk up closer to your subjects using MF camera, ergo shallowed depth of field

if your shooting 75mm with your MF, and then you take a 35mm camera and put a 75mm on it, to get the same framing, you have to take a step back, ergo longer depth of field

had you not moved, you would see less (obvilously) but your depth of field would have been identical.
12-12-2008, 01:22 PM   #12
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okay, if you guys dont get it after this i dont know how else to explain it

the red indicates you moving back while changing formats, but not lenses, to maintain the same field of view, but changing DOF

the black indicates you not moving anywhere while changing formats, but not lenses, and the resultant change in field of view, but same DOF

Last edited by Gooshin; 12-12-2008 at 01:43 PM.
12-12-2008, 01:41 PM   #13
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Thanks guys, for my usage I'll go with 67 has half the dof of 35mm and digi has 1.5x the dof of 35mm. That gives me the mental view I need.
Never understood the whole perspective discussion until now Gooshin. I noticed the difference initially in macro shooting. Had to stop way down to get enough dof with medium format relative to my experience with 35mm at that time. What was included or not via perspective wasn't on my radar. More like how come I can get the entire hoar frost crystal in focus with 35mm, but can't get it with medium format. or similar issue with internal flower parts--damn, got the whole pistil in focus on 35mm but barely just the tip with 67--what gives? Then looked at landscapes... getting a very close foreground and a nearly infinity background both in focus is easy with 35mm using hyperfocal distance, yet is very, very difficult and sometimes impossible with 67 (even using hyperfocal distance scale on the lens). And nowadays, it's stupid simple with digi, though I seldom use digital for landscape work.
But if I consider that a 200 on mf only has the view of a 100 on 35mm, then I'd have to use my distance to equalize the view, and from that distance, the dof will be the same. I actually get it at this point, but for uses like mine, the perspective relevance is far less than the general comparison Misere provided.

For example, the extreme hoarfrost crystal is about the size of a cornflake. 35mm transparency with *200/4 Macro got it nicely. Never could get it with the 67. And that doesn't mean the 67 system is in any way inferior to digi or 35mm. Just has different strengths, dof not being one of them.
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12-12-2008, 01:45 PM   #14
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that is precicely why there is no perfect format

macro and telephoto shooters will favor APS-C

i like narrow DOF and do people shots, usualy trying to fit the whole person in, given typical distance concstraints, i would be very happy with a MF, currently itching for a 5D, but have to make due with a K20D
12-12-2008, 01:57 PM   #15
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Of course there is a flipside--diffraction. Stopping to f22 on digi results in terrible diffraction problems. Stopping to f22 on 35mm results in noticeable diffraction issues. Stopping to f22 on medium format is just another day of shooting. Even f32 can be acceptable with medium format, not so for the smaller digital or 35mm film formats.

So though there may be an increase in acceptable dof with digi, there is a lower limit on how far you can stop down without undue diffraction effect. I noticed this again with macro as f16 on my FA* macro is not nearly as crisp on digi as it always was on 35mm film. I've shot scary sharp images at f22 with 67 macro lens.

Like they say, "if it was easy everyone could do it!"
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