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03-26-2015, 08:24 PM   #1
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When is an APS-C lens not really an APS-C?

Sorry in advance for the newcomer type question that I'm sure has been answered before-

Are all the DA lenses (DA, DA L, DA*, DA Limited) supposed to be APS-C lenses, meaning they project a cropped-sensor sized image circle? Are there any exceptions, such as a DA lens that is marked 50mm but actually produces 75mm on an APS-C body?

Is it true that the engraved focal length on a DA lens is exactly what you'll get when you use that lens on an APS-C body?

I'm confused by some of the lens descriptions on the Pentax website which state 35mm equivalent focal lengths on the DA lenses. What's that about? If the lens is normalized for APS-C, then shouldn't the 35mm equivalent also be the same??? I would expect, for example, a 50mm (marked) DA lens on an APS-C body to produce the exact same FOV as a 50mm FA lens on a 35mm body.

03-26-2015, 08:36 PM   #2
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QuoteOriginally posted by lightbox Quote
Are there any exceptions, such as a DA lens that is marked 50mm but actually produces 75mm on an APS-C body?
Nope. The focal length engraved on the lens is the actual focal length, not a FoV equivalence as on certain compact cameras. So a DA 50mm would have the same focal length and field of view (on an APS-C sensor) as an FA 50mm.

Remember that focal length is an optical properly, and is thus unrelated to the camera's sensor size.

QuoteOriginally posted by lightbox Quote
Is it true that the engraved focal length on a DA lens is exactly what you'll get when you use that lens on an APS-C body?
Yes. The engraved focal length is what you'll get on any camera body, regardless of sensor size.

QuoteOriginally posted by lightbox Quote
I'm confused by some of the lens descriptions on the Pentax website which state 35mm equivalent focal lengths on the DA lenses. What's that about? If the lens is normalized for APS-C, then shouldn't the 35mm equivalent also be the same??? I would expect, for example, a 50mm (marked) DA lens on an APS-C body to produce the exact same FOV as a 50mm FA lens on a 35mm body.
These "equivalences" are massively misleading. In my opinion, the best thing to do is to ignore them entirely.

A 50mm DA lens does not give you the same field of view on an APS-C DSLR as a 50mm FA lens on a film camera. It's only the same if both lenses are used on APS-C.

On APS-C 50mm is a short tele, on a film camera, a normal field of view.

This video should help you out:

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03-26-2015, 08:50 PM   #3
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A 50 mm lens is always going to be 50 mm, regardless of the camera it's mounted on. When comparing to a 35mm camera, the field of view of APS-C is like having a 1.5x TC on the lens. Mount that same lens on Q and it becomes a telephoto. The difference in APS-C image circle vs. 35 mm is the actual diameter of the lens. SInce APS-C is smaller, the DA lenses will vignette on a 35mm camera. It doesn't change the field of view. Several of the DA lenses do work on a 35mm stopped down a little. The camera manufacturers use 35mm of FF as a standard. What makes that very confusing for newcomers is that most have never looked through the viewfinder of a 35mm camera.
03-26-2015, 09:42 PM   #4
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Fantastic explanations (and great video) Thank you both.

So now that I know that there's absolutely no optical difference regarding crop-sensor lenses (except for the smaller image circle), I have to say I am feeling cheated for owning a crop-sensor lens (albeit from Nikon, not Pentax). I know there are tradeoffs such as size, weight and cost, but it seems like it would have been much cheaper for everyone in the long run if they had just saved the R&D costs to make the crop lenses and if every lens a customer could buy would allow them forward compatibility with FF cameras.

But this is why I'm not a businessman!

03-27-2015, 04:13 AM   #5
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Well, they are cheaper, lighter and smaller so there for sure is a market for them. Not everybody will jump on a fullframe camera on first sight.
03-27-2015, 04:26 AM   #6
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QuoteOriginally posted by lightbox Quote
Sorry in advance for the newcomer type question that I'm sure has been answered before-

Are all the DA lenses (DA, DA L, DA*, DA Limited) supposed to be APS-C lenses, meaning they project a cropped-sensor sized image circle? Are there any exceptions, such as a DA lens that is marked 50mm but actually produces 75mm on an APS-C body?

Is it true that the engraved focal length on a DA lens is exactly what you'll get when you use that lens on an APS-C body?

I'm confused by some of the lens descriptions on the Pentax website which state 35mm equivalent focal lengths on the DA lenses. What's that about? If the lens is normalized for APS-C, then shouldn't the 35mm equivalent also be the same??? I would expect, for example, a 50mm (marked) DA lens on an APS-C body to produce the exact same FOV as a 50mm FA lens on a 35mm body.
Focal length marked on a lens is nothing to do with sensor size, focal length is a property of the lens, and magnification only. The same is true for aperture.. 50mm will give you the same image magnification of any subject shot at a fixed length regardless of sensor format.

In lenses, manufacturers identify format with letters. For pentax, DA are cropped sensor lenses, DFA are full frame lenses with digital coatings
03-27-2015, 05:03 AM   #7
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QuoteOriginally posted by Adam Quote
The focal length engraved on the lens is the actual focal length
Though I will point out that some manufacturers play fast and casual with this figure: deviations of 1~3mm from the stated focal length is not uncommon for lenses below 100mm. As for lenses over 100mm there are cases where certain zoom lenses and even primes that fall well short of their stated focal lengths.

Last edited by Digitalis; 03-27-2015 at 09:16 AM.
03-27-2015, 06:52 AM   #8
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QuoteOriginally posted by VisualDarkness Quote
Not everybody will jump on a fullframe camera on first sight.
No arguments, there. I was just wondering if maybe by focusing effort on a single line of lenses for both camera types they could achieve some kind of happy medium? Or is the idea of a small and light FF lenses just a pipe dream?

03-27-2015, 07:19 AM   #9
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QuoteOriginally posted by lightbox Quote
No arguments, there. I was just wondering if maybe by focusing effort on a single line of lenses for both camera types they could achieve some kind of happy medium? Or is the idea of a small and light FF lenses just a pipe dream?
The DA 35, 40XS and DA 50 1.8, FA 31 ltd, FA 43 ltd and FA 77 ltd are all small ,light FF capable lenses.

The focal length of a simple lens, is the distance from the optical centre to the focal point.



If you look at the above and Imagine different size sensor at the focal plane, you can see that the larger the sensor, the more FoV is going to be captured. So the larger the sensor, the wider the FoV. A 50mm is a short tele on an APS-c camera, a Standard lens on a 35mm sensor, and a wide angle on an MF sensor.

Where the equivalence thing works in your favour is weight. A lens projecting a smaller image circle can be smaller, but also, when discussing FoV, when you go to 600mm FF to get the equivalent FoV to 400mm APS-c, the 600mm lens will have a lot more glass, be much more costly, will be a lot heavier, to get the same FoV. At the short end, people claim there is more distortion with APS-c. But the arguments have been theoretical and to my mind inconclusive. In the shorter end, sure a 35mm lens, standard on APS-c is theoretically smaller than a 50, but if you actually hold one or the other, you don't really care. The difference is insignificant.
03-27-2015, 08:15 AM   #10
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For some reason B&H (or Zeiss) feel compelled to give the equivalent length of the Otus 85 even though it costs $4000
QuoteQuote:
It is also compatible with APS-C format DSLRs, where it will provide a 35mm-equivalent focal length of 136mm.
03-27-2015, 09:12 AM   #11
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QuoteOriginally posted by mohb Quote
For some reason B&H (or Zeiss) feel compelled to give the equivalent length of the Otus 85 even though it costs $4000
And? So does Pentax with their $7000 560mm (35mm equivalent 859mm) lens
03-27-2015, 09:22 AM   #12
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QuoteOriginally posted by lightbox Quote
I have to say I am feeling cheated for owning a crop-sensor lens (albeit from Nikon, not Pentax). I know there are tradeoffs such as size, weight and cost, but it seems like it would have been much cheaper for everyone in the long run if they had just saved the R&D costs to make the crop lenses and if every lens a customer could buy would allow them forward compatibility with FF cameras.
Keep in mind that the DSLR was established as king of the digital photography world entirely on the basis of APS-C sensors, before cameras with FF sensors were rolled out. They didn't exist, they weren't foreseen -- or if they were, they were foreseen as costly and specialized items. Entire lens catalogs were filled out with APS-C lenses because that's the sensor size DSLRs used.

If camera makers then had designed their lenses with FF coverage, it would have been only a benefit to those holdouts still shooting 35mm film SLRs. Then all the DSLR users would have felt cheated because they weren't getting the lighter weight, more compact, more affordable lenses that a smaller format made possible!
03-27-2015, 09:42 AM   #13
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QuoteOriginally posted by narual Quote
And? So does Pentax with their $7000 560mm (35mm equivalent 859mm) lens
Do they assume lens buyers are numerically illiterate? If they advertise it as being made for full frame then it seems reasonable to assume that anyone buying it knows the equivalent on APS-c especially if they are spending such large amounts.
03-27-2015, 09:43 AM   #14
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QuoteOriginally posted by Tony Belding Quote
Keep in mind that the DSLR was established as king of the digital photography world entirely on the basis of APS-C sensors, before cameras with FF sensors were rolled out. They didn't exist, they weren't foreseen -- or if they were, they were foreseen as costly and specialized items.
The first DSLR, the Kodak DCS, was a FF camera. It cost $20,000 - $25,000 in 1991, but it was FF. Nikon, looking to compete with the APS-C DCS 300 series (introduced as a "budget model" in 1998), introduced the D1 as the first true DSLR in 1999.

Pentax had prototypes of a FF DSLR, the MZ-D, in 2000-2001. The sensor was deemed too expensive to be profitable at a price point Pentax users would accept and the project was dropped.
03-27-2015, 12:31 PM   #15
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QuoteOriginally posted by mohb Quote
Do they assume lens buyers are numerically illiterate? If they advertise it as being made for full frame then it seems reasonable to assume that anyone buying it knows the equivalent on APS-c especially if they are spending such large amounts.
People who spend large amounts of money are not always smart. Besides, it's a template that's probably applied to every lens they sell. I don't understand why it bothers you.
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