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Old 24-11-2008, 04:05 PM
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Conversion factors 35mm Lens vs DSLR sensor

I have a question relating to 35mm film size (36x24mm) and DSLR sensor size(24x16mm). The cmos sensor is smaller than the traditional film size.
And I have read online that a 75mm SLR lens is equivalent to a 50mm DSLR lens. So for example my old Pentax 50mm F1.4 will be equivalent to a 33.3mm lens on my DSLR body. Could anyone confirm that this is correct?
Secondly, what would be my F-stop for my 50mm if used at say full F1.4 be in equivalent 33.3mm.? And is there a simple way to calculate this.
Many thanks in advance

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Old 24-11-2008, 04:35 PM
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There is no such thing as conversion factor.
50mm lens is ALWAYS 50 mm lens.

However, because CCD/CMOS sensor in most digital cameras is sometimes smaller, the FOV for that particular camera will be smaller (in case of Canon 400D, this is 1.6 times smaller, compared to standard 35mm film camera )

In astro photography, the important parameter is camera scale, which is expressed as pixles/arcsec, or arcsec/pixel. This parameter depends on lens focal length and pixel size.
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Old 24-11-2008, 04:43 PM
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Quote:
Originally Posted by Max Vondel View Post
And I have read online that a 75mm SLR lens is equivalent to a 50mm DSLR lens. So for example my old Pentax 50mm F1.4 will be equivalent to a 33.3mm lens on my DSLR body. Could anyone confirm that this is correct?
Secondly, what would be my F-stop for my 50mm if used at say full F1.4 be in equivalent 33.3mm.? And is there a simple way to calculate this.
Many thanks in advance
What they mean is that if you use this lens on 35mm film and enlarge it to (say) postcard size, then use it on the DSLR and also enlarge it to postcard size, then an object on the DSLR postcard will be bigger than the same object on the film postcard. However the image on the film and the image on the sensor are exactly the same size. If you cut a 24 x 16 mm piece out of the centre of your 35mm film and enlarged it to postcard size it would look exactly like the DSLR postcard in terms of size of objects.

The focal ratio is not affected. An f4 lens is an f4 lens, no matter what sensor you put behind it.

Geoff
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Old 24-11-2008, 04:45 PM
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You have the ratios inverted as well. Since the digital camera with a smaller than 35mm chip will 'crop' the image so to speak, it means that the lenses when compared to 35mm will act like they are of a longer focal length (even though they aren't)

For your Pentax the conversion to compare eqivalent lenses would be:

Focal Length x 36/24 = new focal length

so a 50mm lens on a full 35mm will give the equivalent field of view as a 75mm on your smaller DSLR sensor.
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Old 24-11-2008, 05:03 PM
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Guys...
There is only camera-lens scale, there is no conversion factor.
This term is confusing and misleading, and should never be used.

50mm lens is always 50mm lens. Period.

However, the combination camera-lens has certain field of view, and that parameter is smaller for some DSLR's by factor 1.6, when compared to standard, old-fashioned 35mm camera (and some much more expensive DSLR's with full frame sensor).
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Old 24-11-2008, 05:40 PM
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I completely understand that the focal length of the lens doesn't change, I'm just trying to explain the maths so that someone can work out what lens they might want. If you are used to photography with the nifty fifty on a 35mm and you are wondering what lens on a 1.6 crop camera you would need to use to get the same field of view (or angle of view) then you can use the math example provided to calculate what you need.
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Old 24-11-2008, 05:52 PM
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so a 50mm lens on a full 35mm will give the equivalent field of view as a 75mm on your smaller DSLR sensor.
Wrong way round. A 50mm lens on a big (ie 35mm) sensor is going to give a much wider field of view than a longer focal length lens on a smaller sensor.
A 75mm lens on 35 mm will give (about) the same field of view as a 50mm lens on a DSLR sensor.
Geoff
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Old 24-11-2008, 07:34 PM
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No, not the wrong way around, you just misinterpreted what I said. A 50mm lens from a full frame film camera will seem like a 75mm lens when put it on a DSLR with a smaller (24mm) chip.
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Old 24-11-2008, 07:36 PM
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No, not the wrong way around, you just misinterpreted what I said. A 50mm lens from a full frame film camera will seem like a 75mm lens when put it on a DSLR with a smaller (24mm) chip.
Got it!
Geoff
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Old 24-11-2008, 09:07 PM
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Like I said, confusing.
Forget about crop factor.
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Old 24-11-2008, 11:05 PM
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focal length doesn't physically change, it is a equivalent focal length because the sensor is smaller..

I.e: a 50mm lens on 35mm/full frame DSLR is 50mm..

a 50mm lens on a APS size sensor will have the equivalent FOV of a 80mm lens, on a CANON.. Or a 75mm lens if you are shooting NIKON..

its the same deal with film..


APS DSLR standard lens is 35mm
35mm standard lens is 50mm
6x6cm standard lens is 80mm
4x5 inch standard lens is 150mm

all have the approximate FOV of about 45-50 degrees which is what is 'supposed' to be the closest resemblance to what the human eye sees.. hence standard lens
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Old 25-11-2008, 04:23 PM
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Thanks Everyone

Thanks for all those great answers
I've got a good idea what's going on
Because the imaging area is smaller
it seems like a longer focal length lens
but it obviously is still a 50mm F1.4 for example.
A bit of a waste for that extra field area with
your old lenses.
I feel much enlightened
THANKS
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Old 26-11-2008, 05:26 PM
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Quote:
Originally Posted by Max Vondel View Post
Thanks for all those great answers
I've got a good idea what's going on
Because the imaging area is smaller
it seems like a longer focal length lens
but it obviously is still a 50mm F1.4 for example.
A bit of a waste for that extra field area with
your old lenses.
I feel much enlightened
THANKS
Actually, not quite a waste..
Because if you ever use one of those older, say M42 (much cheaper than dedicated ones) lenses (via 10$ adapter from ebay), you may find in most cases that they are excellent (or adequate) in the centre but increasingly and visibly suffer from coma or CA or other distortions at the corners of the frame.
Now, imagine what it would be if the frame was wider Now, that would have been a waste od sensor area ..

Full frame sensor cameras are up to 5x more expensive.. plus you have to have wider field flattener (again more expensive for full frame) so there is no point to consider smaller sensor as a waste at all.
It is still much wider than those from webcam :-)

Last edited by bojan; 26-11-2008 at 06:11 PM.
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Old 26-11-2008, 09:57 PM
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What an excellent point bojan.
I've been eyeing some of those old Pentax M42 style pentax lenses on ebay.
Mainly for bellows macro work (identifying late roman AE4 coins) but now I can see some other uses! I'd get a nice field with a 135 F3.5 perhaps. With less elements than their modern counterparts I should get more light throughput as well. Maybe bring it down 1 or 2 stops for sharpness as usual.
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Old 27-11-2008, 02:27 PM
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I'd get a nice field with a 135 F3.5 perhaps. With less elements than their modern counterparts I should get more light throughput as well. Maybe bring it down 1 or 2 stops for sharpness as usual.
Yes, definitely.. I am using my 135/F2.8 (Tair-11A) at F4 or 5.6, to get the best sharpness across the frame, and still not having to use very long exposures...
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