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Forum: Pentax DSLR Discussion 09-17-2007, 03:47 PM  
DSLR lens strangeness
Posted By Entropy
Replies: 22
Views: 4,336
Subject image size = size of subject on sensor (regardless of sensor size)

i.e. for a given focal length, subject image size remains constant, but the percentage of the sensor covered by that image depends on sensor size.

As an extreme example - It's nearly impossible to get the "subject in focus, background out of focus" effect with point-and-shoots because the actual lens focal length is so short (in order to achieve a particular field of view on a tiny sensor)
Forum: Pentax DSLR Discussion 09-16-2007, 01:30 PM  
DSLR lens strangeness
Posted By Entropy
Replies: 22
Views: 4,336
Correct. For example, DOF (depth of field) is the same for a given focal length lens regardless of sensor size. The end result is that a shot taken with a 50mm lens with 1.5x crop factor and one with a 75mm lens and 1x crop factor (film or Canon FF) will have the same field of view, but differences in terms of what is in focus. The DOF will be greater for the 50mm 1.5x shot.

This is a well known phenomenon in amateur/semi-professional filmmaking. DV camcorders have much smaller sensors than film cameras, and as a result, much greater DOF. It is generally agreed that while there are many aspects to the "film look" for moviemaking, one of the most important is a narrow DOF, which camcorders just simply cannot replicate without crazy hacks. (If you do some searching, you'll see that movie filmmakers have made adapters that basically project the image of the scene onto ground glass which is then filmed by the camcorder, as opposed to directly filmed, in order to emulate the DOF of film.)
Forum: Pentax DSLR Discussion 09-12-2007, 06:27 PM  
DSLR lens strangeness
Posted By Entropy
Replies: 22
Views: 4,336
This is not true. The focal plane distance for a given mount type is the same whether it is film or digital. i.e. the focal plane distance from the mount is the same for ALL K-mount lenses and cameras.

The difference between film and digital lenses is:
1) Digital sensors are highly reflective, and this can cause problems with lenses that do not have good antireflective coatings. Thus "optimized for digital" really only means "we didn't skimp on our multicoatings like we did for our previous lenses" (For third party lenses this matters, for Pentax lenses it doesn't really since with a few exceptions they are all SMC, which is more than sufficient for this need and why old Pentax SMC lenses work just great on Pentax DSLRs, at least in terms of optical performance.) Third-party designations for this category are Di for Tamron and DG for Sigma. They are listed as "for film and digital" because they have the necessary multicoatings for digital cameras (which do not penalize them on a film camera, and in fact help them) but retain image circle coverage necessary for 35mm film.

An example of an old pre-digital lens that did NOT have sufficient coatings is the first generation Sigma 28-200 3.8-5.6 - It performed fine for years on my family's old PZ-70, but had abysmal contrast when used on a K10D.

2) Digital sensors are often smaller (APS-C is common, but most other manufacturers have multiple sensor sizes available). As a result a lens can be made that has a smaller image circle coverage but still fully covers the sensor. This allows a lens to be lighter and smaller than it normally would be for a given focal length/aperture. (See, for example, the DA 50-200, which is incredibly light and small.) Designators from third-party vendors for these "digital only" (which really only means "small frame only" lenses include DC for Sigma and "Di II" for Tamron.

The focal length and aperture of the lens specified will never be modified from actual values, since this is an inherent property of the lens common across all cameras it might be mounted to. On Pentax cameras, all DSLRs happen to be 1.5x crop factor currently , but other systems have a couple of crop factors available that are body dependent (For example, I think Nikon has a few different sensor sizes that range from 1.3 to 1.6). It wouldn't make sense to put a body-specific specification on a lens that might be used with a different body, even in a case like the Pentax DSLRs where it looks like they will be APS-C for a long time to come and never anything else.

The only time you will see "35mm equivalent" focal length specs put onto a camera lens are either:
1) Clearly labeled as such IN ADDITION to the actual focal length
2) Point-and-shoot cameras with an attached lens that will NEVER be mounted to another camera.
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