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04-03-2008, 04:37 PM   #16
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QuoteOriginally posted by Geekybiker Quote
But now we have a crop of cameras using at least 5 formats in DSLR's alone. (ff, 1.3, 1.5, 1.6, 2) Not to mention the whole host of PS cameras that use assorted sensor sizes. Its not like film where there were few formats to compare. There has to be some sort of useful standard to compare the performance of lenses across models. Granted 35mm equivalent isn't ideal. Maybe listing the magnification from normal perspective? Of course that breaks down with interchangeable lenses where the only constant is focal length. Then using some sort of conversion factor is the only way to compare performance.

How would you list lenses such that they are easy to compare across a different platforms?
See my post on the real issue of the enlargement ratio. This might depend on the individual lens and camera makers but here is what I would do.

the 4/3 system is unique (crop factor of 2), and any lens made for that system should quite simply be marked with correct DOF for that camera. I think it is that simple.

the same goes for any lens that is a full frame lens. That goes for my Sigma APO 70-200 F2.8 EX and Tamron 28-75mm F2.8 Di because I could quite easily (and have in the past) put them on my PZ-1 and shot a frame or two.

for lenses that are meant for ASP-C sensors, like Tamron's Dii series, or Sigma's DC series, then these should be marked with DOF markings in the most popular senmsor size which is the ASP-C sensor. The slight error in DOF markings between the 1.3 1.5 and 1.6 crop factors is not that significant

People buying a full frame lens should accept that the DOF markings are not accurate for an ASP-C sensor.

04-03-2008, 04:42 PM   #17
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Thanks - but I beg to differ

QuoteOriginally posted by Geekybiker Quote
The thing is the DOF subject is really pretty complex, and what affects DOF depends heavily on what we are choosing to stay constant.

35mm equivalent is the best place to start IMHO. It will produce identical framing, perspective and subject distance. Perhaps FOV is a better term than focal length for comparisons between systems.

Main subject framing is iffy. You'll have extremely different pictures from a FF using a 6mm lens to a 1/1.8" using that same lens.

Really there are 4 things that play a part depending on what you chose as a constant
Aperture
Subject distance
Sensor size
focal length

Anyhow, I suggest you read through PDL's links.
Thanks for the suggestion that the links be read in order to gain a bit more understanding.

However, I do disagree about the idea of equivalent focal length. If you do read the information provided by the links, you will not find the use of the word equivalent anywhere.

DoF is involved with the focal length of the lens - period - there is no magic 'factor' necessary. FoV also has no place in the arguement. DoF is a function of:
Focal Length
Aperture
sensor size
Subject distance is not strictly a consideration except when using DoF have the subject placed within the DoF as calculated. I use DoF - and the calculations as defined via DoF calcualators - to set the subject distance to bring either the forground or background into focus.

At no point do I use the terms 'equvalent' or 'FoV' to determine DoF. I again suggest that for this discussion - the terms equivalent and FoV have no place, they only confuse.

The Elitist - formerly known as PDL
04-03-2008, 04:49 PM   #18
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QuoteOriginally posted by Lowell Goudge Quote
Although people seem to have danced all around the issue, they have not hit the issue directly.

Depth of field is dependant upon two things, the first is the lens and its apature, (and this is camera and sensor independant) and the second is the magnification ratio of the print.
This is true, but it is accounted for in the DOF calculators (and the DOF pages I've read) and the smaller sensor still has greater DOF for the same FOV. So if you'd rather attribute that to the change in focal length than sensor size, that's fine. Its all semantics at that point I think.
04-03-2008, 04:55 PM   #19
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QuoteOriginally posted by PDL Quote
DoF is involved with the focal length of the lens - period - there is no magic 'factor' necessary. FoV also has no place in the arguement. DoF is a function of:
Focal Length
Aperture
sensor size
Subject distance is not strictly a consideration except when using DoF have the subject placed within the DoF as calculated. I use DoF - and the calculations as defined via DoF calcualators - to set the subject distance to bring either the forground or background into focus.

At no point do I use the terms 'equvalent' or 'FoV' to determine DoF. I again suggest that for this discussion - the terms equivalent and FoV have no place, they only confuse.

The Elitist - formerly known as PDL
Wait, didn't I just say that exact thing? I have read all those links and more on this subject. In terms of lens physics or a DOF calculator 35mm equivalent has no place, sure. However as a reference so photographers know what you're talking about its a worthwhile convention. (or for marketing or whatever.) Saying I took a shot with a 300mm lens (35mm eq) is much more useful in normal conversation than saying I took it with a 150mm lens on a olympus 4/3 camera with a 2x crop factor. Its more concise and doesn't require the reader to have intimate knowledge of the equipment you're using. *shrug*

04-03-2008, 04:56 PM   #20
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QuoteOriginally posted by Geekybiker Quote
But now we have a crop of cameras using at least 5 formats in DSLR's alone. (ff, 1.3, 1.5, 1.6, 2) Not to mention the whole host of PS cameras that use assorted sensor sizes. Its not like film where there were few formats to compare. There has to be some sort of useful standard to compare the performance of lenses across models. Granted 35mm equivalent isn't ideal. Maybe listing the magnification from normal perspective? Of course that breaks down with interchangeable lenses where the only constant is focal length. Then using some sort of conversion factor is the only way to compare performance.

How would you list lenses such that they are easy to compare across a different platforms?
Focal length is Focal length - that's all that needs to be said.

Do you suggest that you take a P&S with a 1/8th inch sensor and do DoF calculations based on a 135 format sensor? I beg to suggest that this is just confusing the issue. Focal length does not change - regardless of sensor size - why not just use it and move on.

To Lowell - yes the CoC changes due to the size of the sensor. This is explained reasonably well in the DoFMaster help file. In my primative calculations I have settled on a CoC of 0.025 for my K10D/*ist Ds where the guys at C*non have suggested that 0.030 is the default for 35mm (135 format). So yes, you need to come up with what you are comfortable with in your use of DoF calculations - but let's keep the idea of equivalent out - it is doing nothing but confusing the issue.

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04-03-2008, 05:04 PM   #21
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no - not the same thing

QuoteOriginally posted by Geekybiker Quote
Wait, didn't I just say that exact thing? I have read all those links and more on this subject. In terms of lens physics or a DOF calculator 35mm equivalent has no place, sure. However as a reference so photographers know what you're talking about its a worthwhile convention. (or for marketing or whatever.) Saying I took a shot with a 300mm lens (35mm eq) is much more useful in normal conversation than saying I took it with a 150mm lens on a olympus 4/3 camera with a 2x crop factor. Its more concise and doesn't require the reader to have intimate knowledge of the equipment you're using. *shrug*
A 300mm lens on a 2x crop factor provides the same FoV as a 600mm in 35mm. Not 150 - you multiply by the crop factor you do not divide.
EDIT:
OK so I mis-read the post - in my opinion telling someone about the equivalent focal length is just going to add to the confusion. A large number of people who are moving into DSLR's now do not have any relationship to 35mm film photography. Just tell them what you took the image with - the act to trying to explain what equivalent means will do nothing but provide you with the oportunity to see even more people with 'cow lost in a snowstorm' looks. When people were using Instamatics (126 film or 110 film) did anyone really ask what the equivalent focal length was compaired to 35mm? When I dig out the 4x5 and its 90mm lens - do I convert it to a 35mm equivalent? No - I did/do not. Equivalence is passe and should not be addressed when discussing DoF.
END EDIT


My FA 50mm on my SF-1 is a "normal" lens - or within the convention for "normal", while on my K10D it provides the same FoV as a 75mm lens, just getting into the 'telephoto' range (the equivalent discussion). However, the focal length does not change. It is the focal length that is part of the DoF calculation not the equivalent or FoV - both terms are not relevant.

The Elitist - formerly known as PDL

Last edited by PDL; 04-03-2008 at 05:18 PM. Reason: Please note the edit
04-03-2008, 05:07 PM   #22
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For a given focal lenght, depth of field is the same, wether the sensor size is APS-C or FF doesn't make any difference. If you take a lens with depth of field scale, you can rely on the scale, no matter what the sensor size is. The only thing that will make a difference is the enlargement ratio, since the DOF scale was made taking into consideration what is acceptable sharpness to the average people. Things would be a lot simpler if people would forget the "crop factor", since this is an expression used to make a comparison with 35mm film camera. A 50mm lens is a 50mm lens. Putting it on a FF sensor or APS-C sensor doesn't change a darn thing. People should stop thinking in terms of 35mm cameras and start thinking "their" sensor size. Nobody talks about "crop factor" when they talk 2 1/4 square or 5X7 film cameras. Why do it for digital, then? On APS-C, 30mm (or so) is normal, longer is telephoto, shorter is wide angle. That's all there is to remember.
04-03-2008, 05:17 PM   #23
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I am afraid I don't care for all that math. There is a little spring loaded thingy on the on switch of my K10D that lets me look at my depth of field through the finder. That is the same system I have used with my Asahi Pentax, KX, ME, SF-1 and MZ-S. Although I had to do some cheating on some of them and press the lens mount release and turn the lens just far enough that it didn't fall off, but far enough to let the aperture close. It was much easier with the Asahi Pentax, the KX and the MZ-S, and now the K10D.

04-04-2008, 01:22 AM   #24
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QuoteOriginally posted by pixelsaurus Quote
Hear,hear.
The nitwit(s) that dreamed up the"crop factor/equivalent focal length" , a marketing ploy I guess, should be evaporated. It has done nothing but sow the seeds of confusion. When I was a 35 mm user ( I started photography in 1969), there was no confusion relating to lens focal lengths re 35mm, MF and LF cameras. I'm certain the concept of equivalent focal lengths was of no consequence to most photogs. My concern these days is that many newbie photogs (digital) really do have a lack of understanding of basic photographic and optical principles.
the thing is people are used to thinking of a 50 mm lens as normal and the longer a telephoto and the shorter a wide angle now I go and buy a camera that has a sensor that is 1/4.5 the size of a 35 mm film frame it has on it a lens of 6.2-66.7mm , now as I'm not told the details of the sensor I'll hav to start from scratch figuring out what lengths achieve what, we used to live in a world where everything was either medium format or 35mm now with every tom dick and harry making a different size sensor you never know what ya lens is going to do unless you know either the size of the sensor or the 35mm equi. size of the lens if I walked into a shop and picked up my fuji with written on it 6.2-66.7mm I would probably think I have a super wide angle lens on it only to discover it acheives the same as a 28-300 mm in practice,

now if depth of feild is the same on any size sensor (because it is related to the lens size) what I need to acheive in framing with my K10D on a 18 mm lens I can acheive on my fuji with a 6.2 mm lens hence the fuji will have more depth of field while producing the same framing
04-04-2008, 02:44 AM   #25
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DOF

QuoteOriginally posted by Geekybiker Quote
Really there are 4 things that play a part depending on what you chose as a constant
Aperture
Subject distance
Sensor size
focal length

Anyhow, I suggest you read through PDL's liks.
Suppose you have an image on your monitor taken with a perfect lens, very high resolution sensor, showing in screenfill mode. You can determine what is acceptably sharp for you. That is the DOF for that image.
Now use the crop tool to reduce the area you're viewing to ~66% or your screen.
You have done the same as using a 1.5X crop sensor. DOF stays exactly as before.
What did change is the FOV for the cropped part.
04-04-2008, 03:38 AM   #26
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QuoteOriginally posted by baw Quote
Suppose you have an image on your monitor taken with a perfect lens, very high resolution sensor, showing in screenfill mode. You can determine what is acceptably sharp for you. That is the DOF for that image.
Now use the crop tool to reduce the area you're viewing to ~66% or your screen.
You have done the same as using a 1.5X crop sensor. DOF stays exactly as before.
What did change is the FOV for the cropped part.
yea it increaes if you took the same picture straight out of the cmaera it would have been taken with a lens 1.5 X the length of the one you used so would have had less DOF
04-04-2008, 03:48 AM   #27
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DOF

QuoteOriginally posted by simons-photography Quote
yea it increaes if you took the same picture straight out of the cmaera it would have been taken with a lens 1.5 X the length of the one you used so would have had less DOF
Sure, and if you use a higher F-stop number the DOF will increase.....

The whole point is by keeping all other variables the same just changing the crop doesn't change the DOF. I can't express it any clearer than this.
04-04-2008, 04:43 AM   #28
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QuoteOriginally posted by baw Quote
Sure, and if you use a higher F-stop number the DOF will increase.....

The whole point is by keeping all other variables the same just changing the crop doesn't change the DOF. I can't express it any clearer than this.
precisley and because a different length lens is used according to the sensor size to keep things constant being a different focal length (and producing the same effect) there will be a difference DOF because different size lenses are used in relation to the sensor size they work on a smaller sensor and lens system will have more DOF than a larger sensor and equivalent size lens
04-04-2008, 09:56 AM   #29
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DOF

QuoteOriginally posted by simons-photography Quote
precisley and because a different length lens is used according to the sensor size
Did you actually read my post?
I only cropped the image on screen, doing exactly the same as using a smaller sensor would.
So the same lens, same aperture, same focus distance, same viewing medium, same viewing distance, same EVERYTHING, except cropping the image.

Try using sentences in your posts, and perhaps a carriage return every now and then. Your post doesn't make much sense to me as it is.
04-04-2008, 11:03 AM   #30
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QuoteOriginally posted by baw Quote
Suppose you have an image on your monitor taken with a perfect lens, very high resolution sensor, showing in screenfill mode. You can determine what is acceptably sharp for you. That is the DOF for that image.
Now use the crop tool to reduce the area you're viewing to ~66% or your screen.
You have done the same as using a 1.5X crop sensor. DOF stays exactly as before.
What did change is the FOV for the cropped part.
Yes, this is true. Same lens cropped the same yields the same DOF independant of sensor (Though in the real world you'd have very different resolutions after the crop)

Like I said, those 4 factors define DOF depending on which ones you hold constant. You'll notice that its also what DOF calculators ask for. I think PDL touched on it earlier. Sensor size matters primarily due to the magnification factor to print. Acceptable CoC size varies with sensor size.

Anyhow I just feel like we're going around in circles at this point with most of us saying the same thing in different ways.
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