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02-08-2011, 10:37 PM   #1
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So... Many... Lenses

I'm on a quest for an HSM prime/portrait lens for my K-7. Because K-7 is APS-C, a 30mm lens is 47-50mm. Did I calculate that right?

Is 55mm really 55mm when talking about a digital lens like DA* 55mm f/1.4??
Or is it seen as focal length of ~85mm from the 1.5x crop factor of the sensor?

Its confusing in the reviews about focal length on the lens and the focal length to the sensor with these prime lenses...

The DA 31mm limited seems to be the holy grail of lenses, but it's the same price as a new K-5 ($1500)

I found a more reasonably priced Sigma 30mm f1.4 EX DC HSM ($450)
But I am reading contradictory information about that lens, some say it doesn't have HSM for the pentax mount even though the specs say otherwise.

Happy Wednesday!
-C

02-08-2011, 10:46 PM   #2
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A 55mm lens is a 55mm lens no matter what body it's on. But smaller frames (film or sensor) see less of the image circle it projects -- the edges get chopped off. Moving a lens to another body does not change the lens. Oh yeah, forget you ever heard the term crap factor, it's only meaningful if you're an experienced 35mm film photographer transitioning to APS-C cameras. Otherwise it's just a confusing obscenity.

What do you mean by 'portrait' lens? Formal studio portraits are one thing; grabbing impromptu shots of people in the wild is something else entirely. Portraits can be shot with any focal length -- ultrawide for context, ultralong for safety (like a Mafiya funeral), different focal lengths for full-body and head-and-shoulders and ID facials or whatever. No matter the frame size, 70-100mm gives nice roundness to features; 135-200 flattens the features a bit; shorter lenses invite you to work closer. So that partly depends on the comfort zones of you and your subjects.

Suggestion: Get a cheap old 35-135 manual zoom, something around there. Or use a kit 18-55 and a cheap old 70-200-ish, like a Sears for US$15. Get a subject or three. Shoot them from various distances with various focal lengths. See where you and they are comfortable, where you get the kind of shots you want. Take notes. See what focal lengths work for you, before spending more money. You'll probably find that 30mm makes you work too close, and 135+ takes you too far away. And in controlled shooting setups, autofocus is unnecessary.

Last edited by RioRico; 02-08-2011 at 10:58 PM.
02-08-2011, 10:53 PM   #3
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50mm might be a tad short for head-and-shoulders portraits. It's a 75mm equivalent with the crop factor. In the old days, portrait lenses were in the 80-105mm range. So, you can get a Sigma or other 70mm macro (105mm equivalent) and have a dual-purpose lens.
02-08-2011, 11:08 PM   #4
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QuoteOriginally posted by chattphotos Quote
I'm on a quest for an HSM prime/portrait lens for my K-7. Because K-7 is APS-C, a 30mm lens is 47-50mm. Did I calculate that right?

Is 55mm really 55mm when talking about a digital lens like DA* 55mm f/1.4??
Or is it seen as focal length of ~85mm from the 1.5x crop factor of the sensor?

Its confusing in the reviews about focal length on the lens and the focal length to the sensor with these prime lenses...

The DA 31mm limited seems to be the holy grail of lenses, but it's the same price as a new K-5 ($1500)

I found a more reasonably priced Sigma 30mm f1.4 EX DC HSM ($450)
But I am reading contradictory information about that lens, some say it doesn't have HSM for the pentax mount even though the specs say otherwise.

Happy Wednesday!
-C
The term you want to use is Field Of View when thinking of lenses in 35mm format terms (compared to APS-C). I'm not going to argue about what makes a good portrait lens but the DA*55 is quite up to the task if you want to spend the money.

There is no DA 31, you probably mean the FA31mm f1.8. Yep, it's pricey, it's the holy grail of Pentax Primes, and if you can stand used, they can be had for the neighborhood of $900. Check the Pentaxforums marketplace.

The Sigma 30mm f1:1.4. I know nothing about those lenses except this. The 30 has been around a long time. It was originally produced without HSM, that version is discontinued, and now is produced With HSM. If I'm not mistaken, they also offer a 50mm.



02-08-2011, 11:24 PM   #5
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QuoteOriginally posted by chattphotos Quote
Its confusing in the reviews about focal length on the lens and the focal length to the sensor with these prime lenses...
Don't think in terms of FL's. A 100mm lens, for instance, will project the same angular cone of light toward the sensor no matter what body you mount it on.

The difference is, the APS-C sensor will only intercept half the area of the projected cone compared to a full frame sensor (or a 35mm film frame) and therefore it will produced a 'cropped' image similar to what a longer telephoto lens would do on the full frame sensor.

H2
02-08-2011, 11:53 PM   #6
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Is crap factor a scientific term?

...that made my night
02-09-2011, 02:17 AM   #7
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Coming back to the original post:
- For portrait, many use some lenses with focal lenses about 50-100mm.

- The infamous FA31 mm Ltd is not really a portrait lens. I have it. It is a great lens even in low light, but I prefer my Voigtlander Nokton 58mm f1.4 for portrait.

- A longer focal length (~100mm) allow you to shot from further away, but the 50-55mm is really considered the typical portait focal length.

- A number of 50-55mm have a large aperture (eg f1.4) to allow shooting without a flash even if the light is below average. This is an definite advantage if you do not want to use the flash, often too intrusive. These large aperture lenses are also called fast prime lenses.

- A basic choice will be MF or AF. There are some very high quality MF fast primes, including the Carl Zeiss 50mm f1.4 and Voitglander Nokton 58mm f1.4. For AF, the Pentax FA50mm f1.4 is an obvious choice, together with the Pentax 55mm SDM.


By the way, the FA31mm is worth about $1,0000 new. Shop around. You should not pay $1500 for it. It is a great lens, definitely: I bought one from B&H for Xmas.

Hope that the comments will assist.
02-10-2011, 04:08 AM   #8
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QuoteOriginally posted by calculator01 Quote
Is crap factor a scientific term?

...that made my night
Thank you, thank you. [/me buffs nails]

Actually it's a marketing term, same as "35mm equivalence" and "10x zoom" and "compassionate conservative". It promotes the misconception that the act of moving a lens somehow changes its qualities. Sort of like advertising that consumption of some specific brand of burger or cola will change your personality. It's bogus. [/me swigs generic soda, remains /me]
_______________________________________________________________________________

Back to the question, sorta: A 50mm lens on APS-C gives the same field-of-view (FOV) as a 75mm lens on 135/FF, or 120mm on 645, or 175mm on 6x9, or 14.5mm on a P&S with a 1/1.8" sensor. But we know that a 14.5mm lens does NOT behave like a 175mm lens. DOF and dimensionality are radically different.

Back in the day, I had a 135/FF (half frame) Olympus Pen-FT SLR, frame size about the same as our APS-C dSLRs. I bought a cheap (of course) Spiratone 400/5.6 tele on a T2 adapter. I thought, OH BOY, NOW I HAVE A 600MM LENS! I quickly learned my mistake -- it's just a 400mm lens, with the sides of the image cut off by the smaller frame, that's all.

Also back in the day, I worked with 135/HF, 135/FF, 6x6, 6x9, and 9x12 cams, sometimes all on the same day. Some lenses could be moved between cams. My colleagues and I never used the term crap factor -- it hadn't been coined yet. We just quickly learned that various focal lengths behaved in various ways on various cams.

And that gets back to portrait lenses. On 125/HF-FF and 645 and 6x6 and even 6x9, I found that the best focal length for faces was 75-80mm, f/2.8-3.5. More roundness than 120-135-150mm; better separation-from-context than 45-50-55mm. That separation, partly a function of DOF, can be approximated: for 80/2.8 on 135/FF, use 50/1.7 on APS-C at the same working distance. But for the roundness, use a 77-80-85mm lens at f/2.8. Try it and see.

02-10-2011, 06:28 AM   #9
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QuoteOriginally posted by RioRico Quote
Back in the day, I had a 135/FF (half frame) Olympus Pen-FT SLR, frame size about the same as our APS-C dSLRs. I bought a cheap (of course) Spiratone 400/5.6 tele on a T2 adapter. I thought, OH BOY, NOW I HAVE A 600MM LENS! I quickly learned my mistake -- it's just a 400mm lens, with the sides of the image cut off by the smaller frame, that's all.

.
Quite true, but cutting those sides away makes an important. I remember (really back in the day--my childhood) getting my first Instamatic and a Sears darkroom set which made something like 4x6 prints on a little horizontal enlarger. I would pick a subject through the viewfinder which was my center of my attention, but it was so insignificant unless I cropped it as my braiin had done which I looked though the viewfinder.
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