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09-09-2007, 02:30 PM   #1
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Why do Pentax DA lenses still use 35mm focal lengths ?

If the DA series is manufactured specifically for the chip on Pentax digital cameras, why retain the focal length reference to full frame 35mm film cameras? They will not work on a full frame camera anyway because they can't cover the chip.

Will this standard still remain in the future when camera chips possibly exceed the full frame holy grail too? Is there no other constant that can be used, similar to F stops that doesn't vary from chip to chip?

I suppose that if we still retain "horsepower" as a legitimate measurement for engine power, that the 35mm reference will be with us long after the last piece of film is made.

Just a drizzly Sunday afternoon rumination seemingly not destined to reach any conclusion.

09-09-2007, 02:40 PM   #2
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focal length is focal length.

when you put your 50-135 or whatever onto your k10d or whatever the rear glass element will be 50-135mm away from the sensor.
a 50-135mm on a 26mm digital seonsor behaves like a 75-197.5 on a 35mm film camera because of the properties of the light leaving the rear element of the glass and spreading wider to cover the 35mm film. therefore you have a lens behaving as though it were a wider lens.

im not sure if the distance from the mount to the sensor is different than the distance from the mount to the film, but that would also be a factor.
if the DA* 50-135 is on a film camera and because of the build of the body the mount and the rear glass element are farther away from the film, that would be a direct factor in lens behaviour.

this is kind of a vague post, but what the hey, i felt like typing something.

mitch
09-09-2007, 02:55 PM   #3
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Focal length is focal length, correct. The mount to sensor or film-plane distance with the Pentax K-mount is 45.5mm. So it's the same on both formats. That's why we can use old M42 and early K mount lenses on the new cameras without a spacer.

What gets the whole issue confused is the sales terminology which seems to say a 150mm lens somehow magnify's the image more. It doesn't, it just crops it more because the sensor is smaller than the size of the film.
09-09-2007, 04:52 PM   #4
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Aha..I understand it now.

So the terminology "equivalent to" is in fact that. Is there a physical reason why the sensor could not be moved forward to produce the same FOV of the 35mm film plane further back? Would that put it into the glass, or is it a focus problem requiring the "film plane" to remain where it is?.

Maybe I should forget all this and go out and take some photos. Becoming an armchair photographer so early in my career.


Last edited by DMH; 09-09-2007 at 05:02 PM.
09-09-2007, 05:39 PM   #5
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There are a lot of confusing statements here.

Lets be clear. Focal length, is for a single lens the distance from the lens to the focus point for an object at infinity.

Most lenses today are complex lenses, (many Elements) and as a result the "focal length" is really the "equivalent' focal length to a simple lens, in terms of the ratio if subject size to image size at any distance of subject and focal point. This does not change when you go to digital format, this is a physical property and nothing more.

F Stop is the ratio of diameter to length. (again on simple lenses) and is a measure of the light capturing ability of the lens. Again this is a physical property. on long lenses it is usually very easy to calculate because it is the focal length divided by the front element diameter. On SHorter lenses this is not always the case because the diameter (especially of filters) may be very large to keep them away from the field of view and prevent vignetting.

The third physical characteristic of a camera lens is the coverage, which relates to the diameter of a circle which is evenly illuminated at the focal plane. Digital lenses cover a smaller circle (because the sensor is smaller) than a 35mm film frame or full frame sensor. Essentually digital cameras crop the frame using only the center 2/3 (for ASP-C sensors) or about 1/2 (for 4/3 sensors). This allows for some reductions in size and weight since diameter of certain elements can be reduced within the lens, but does not change the focal length or F stop at all.

With respect to the "film or sensor plane" and the lens, a specific definging criteria for single lens reflex cameras is the space required to swing the mirror. Since most digital manufacturers started out as film camera makers, and there is some degree of compatibility between lenses from both film and digital, the lens mount to film plane has remained fixed.

On a view finder camera, or a p&s where image viewing is done off the sensor, there is no mirror. This allows for smaller sensors and smaller (lower focal length lenses) to get an image projected on the sensor. It all comes down to cost. these cameras are lower cost (usually) because the lenses are smaller, sensors are smaller, etc... BUT the image quality suffers, especially with noise at high ISO

Last edited by Lowell Goudge; 09-09-2007 at 05:45 PM.
09-09-2007, 05:41 PM   #6
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QuoteOriginally posted by DMH Quote
So the terminology "equivalent to" is in fact that. Is there a physical reason why the sensor could not be moved forward to produce the same FOV of the 35mm film plane further back? Would that put it into the glass, or is it a focus problem requiring the "film plane" to remain where it is?.
The distance of the sensor to the lens mount flange has nothing to do with the lens focal length, the image size, or the field of view. That does affect things like focus, but that is not what you are talking about.

A 35mm film frame is about 24x36mm.

The digital sensor in the Pentax DSLRs is about 18x24mm. That's about 2/3 the size of a 35mm film frame.

So, for a given lens, you only capture the central 2/3 of the image projected on the focal plane, as compared to a 35mm camera. Hence the "crop factor." So the 35mm film camera has a field of view that is 1/3 wider (and higher) than the digital SLR with a 18x24 sensor when the same lens is used.
09-09-2007, 07:55 PM   #7
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QuoteOriginally posted by DMH Quote
Will this standard still remain in the future when camera chips possibly exceed the full frame holy grail too? Is there no other constant that can be used, similar to F stops that doesn't vary from chip to chip?
There is another arguably better constant: horizontal field of view in degrees. But people just aren't used to thinking that way.

Also: It's unlikely that (not counting the mythical digital 645 medium-format) dSLRs will exceed full-frame, because current (and past) lenses wouldn't cover the image circle (at least not without severe vignetting). And any new lenses would have to be bigger and heavier -- the opposite of the general trend.
09-10-2007, 05:41 PM   #8
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QuoteOriginally posted by DMH Quote
...I suppose that if we still retain "horsepower" as a legitimate measurement for engine power...
Well, people that refuse to come into the 20th century do anyway!
Kilowatts is the International unit of power.

09-10-2007, 06:37 PM   #9
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QuoteOriginally posted by DMH Quote
I suppose that if we still retain "horsepower" as a legitimate measurement for engine power, that the 35mm reference will be with us long after the last piece of film is made.
Be careful here, I think the metric comission has rounded off horsepower, It used to be 1 hp = 746 watts, but I think reciently (since I went to university) they have standardized it at 750 watts
09-11-2007, 03:17 AM   #10
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I'm starting to understand the problems with standardization now.

Camera megapixels will continue to grow. Maybe not exponentially with respect to time like computer CPUs but the density and size will keep adjusting to reflect technological advances. Therefore, neither the pixel density or chip size will remain constant and therefor the associated FOV of each lens will vary.

At some point I suppose I will have to forsake the connection developed during my film camera days, of a particular focal length to it's 35mm FOV.

When and if sensor densities exceed what we can use, ie. see with our eyes (can't believe I wrote that ) then we could possibly carry one really good prime and resort to digital zoom to adjust field of view. It all sound so inelegant and opposed the very nature of photograph. But, so did digital cameras when they came out.

I work in the film industry and i'd say 99% of movies are shot on film stock. Cinematographers I've talked to dislike the ability to "see into the shadows" that digital sensors give. The wide exposure latitudes of digital removes the mystery and requires a lot more detail and a higher level of detail on the sets we build.

Still photography seems to benefit from the additional information whereas in movies it can be a distraction.

But I stray from the topic.
09-11-2007, 01:09 PM   #11
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QuoteOriginally posted by DMH Quote
When and if sensor densities exceed what we can use, ie. see with our eyes (can't believe I wrote that ) then we could possibly carry one really good prime and resort to digital zoom to adjust field of view. It all sound so inelegant and opposed the very nature of photograph. But, so did digital cameras when they came out.

I work in the film industry and i'd say 99% of movies are shot on film stock. Cinematographers I've talked to dislike the ability to "see into the shadows" that digital sensors give. The wide exposure latitudes of digital removes the mystery and requires a lot more detail and a higher level of detail on the sets we build.

Still photography seems to benefit from the additional information whereas in movies it can be a distraction.

But I stray from the topic.

Actually, my understanding is that the resolution of current sensors already exceed our eyes capabilities. Look at a 10MP picture and stand the same distance, with a good 50mm lens, the picture will have better resolution details. For example, I can notice things on the picture (zoomed in on my monitor) that I did not notice at the scene (since we're unable to zoom in naturally).

Our eyes has a much greater dynamic range compared to pictures, that's why we're able to create 32bit HDR images and still notice all the colors. Digital sensors have a long way to go in that respect.

A big advantage that film has over digital sensors is the ability to capture greater dynamic range (which would allow for MORE light or shadow detail within the same picture). I've thought movies hide shadows or blow out highlights via post processing, not because it didn't exist on film. The transition to digital does seem slower for Cinematographers, but I've seen professional grade triple CCD cameras for a long time now. Those new HD cams are pretty impressive as well from what I've seen.
09-11-2007, 01:57 PM   #12
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My understanding is that the dynamic range of digital sensors on current DSLRs is greater than on "standard" film. I checked my facts and found this exhaustive comparison. Clarkvision: Dynamic Range of an Image
Check it out, the conclusion and the tests seem to back up my statement but I must be honest, I just glanced at the tests and read the conclusion. Film seems to have greater range in other areas according to the test.

Aside from the dynamic range, cinematographers seem to prefer film because they can "steer" it in a particular direction more easily than digital. It's about exclusion. Or they may just be more used to it. This allows them to follow the intended script narrative by excluding or selectively including. Blowing out the highlights for example allow you to have a window without a backdrop. We're so used to it that it seems natural. We even create "blown out" backdrops that are very soft to mimic the shallow depth of field of film. It's crept into our visual vocabulary and will be hard to replace.

The 2004 film starring Tom Cruise as a hitman, Collateral was shot on digital. It allowed the use of available light and was quite noisy and gritty, a look that worked well with the subject matter. There seemed to be little correction for different colour temperatures which also worked to make it feel real. Point is, that digital seems to be a specific look in motion pictures.

As I said in an earlier post, the inclusiveness of digital seems to help still photograph. Our vision is cumulative over time and what we imagine we see is seldom reflected in the fraction of a second it takes for a photo. Or maybe motion pictures gain by compressing time and stills gain by expanding it or freezing it.
09-12-2007, 01:49 AM   #13
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Why do Pentax DA lenses still use 35mm focal lengths ?

The question that started this thread is flawed. The DA lenses DO NOT use 35mm focal lengths.
Here is a quote from the DA 18-55 mm lens test on PhotoZone Pentax SMC-DA 18-55mm f/3.5-5.6 AL - Photozone Review / Quick Test Report
"The Pentax DA 18-55mm f/3.5-5.6 AL is one of the kit zoom lenses currently sold combined with the K10D or the Pentax budget DSLRs. It is a DA series lens so it is dedicated to the APS-C format with a reduced image circle. Its field-of-view is equivalent to 27.5-84mm in the 35mm format."

DA lenses are not using 35mm focal lengths. End of story.
09-12-2007, 02:45 AM   #14
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I thought that had been established.

That is that focal length is focal length no matter what size film or sensor is used. The field of view (FOV) changes, not the focal length which is the distance from the film/sensor plane to the point of focus to the rear principal plane of the lens. Actually it only applies when the lens is focused on infinity. The quoted focal length actually changes when focused on an object closer than infinity.

Am I being too thin skinned or is "flawed" a little undiplomatic?
09-12-2007, 03:13 AM   #15
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In the "old money" 50mm was the mid-point between W(ide) and T(ele) so I tend to think:
17mm=3W, 20mm=2.5W, 28mm=1.8W
70mm=1.4T, 100mm=2T, 300mm=6T

With APS-C mid-point is 33mm therefore:
17mm=2W, 20mm=1.7W, 28mm=1.2W
70mm=2T, 100mm=3T, 300mm=9T

It's simpler to say 70mm in APS-C is 2x Tele rather than it's equivalent to 100mm focal length in 35mm system.
It also readily explains 28mm= 1.2x Wide is no longer very wide.

Last edited by Kguru; 09-12-2007 at 05:51 AM.
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