Why All the Concern with Lens Speed?

This is sort of like the megapixel race. 22 is better than 16 better than 12 better than 10 MPix and so on. Is an f1.4 lens better than an 1.8, 2.0, 2.8 etc..?

I've barely ever taken a decent shot at f1.4 before and only have an old Super Takumar 50/1.4 that is that fast.

There's an entire market of existing Pentax users who will jump all over this lens once they think about what they really need (assuming it's pretty good). It's a great way to introduce newer users (gained the last few years) to the Pentax prime experience (not you Prime) and hopefully they'll then spend some real $ on some Limited's.

Folks keep in mind this forum includes ALL Pentax DSLR's. Pro user's to beginners. There is a wide range of users with a wide range of needs. A relatively fast inexpensive, plastic general purpose prime will fill the needs of many average users. It might be insulting to some advanced users but you old fellas aren't exactly a growth industry for Pentax.

To me the biggest issues are DOF and brightness of image in viewfinder.
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wrote:

seeblue said:

This is sort of like the megapixel race. 22 is better than 16 better than 12 better than 10 MPix and so on. Is an f1.4 lens better than an 1.8, 2.0, 2.8 etc..?

I've barely ever taken a decent shot at f1.4 before and only have an old Super Takumar 50/1.4 that is that fast.

There's an entire market of existing Pentax users who will jump all over this lens once they think about what they really need (assuming it's pretty good). It's a great way to introduce newer users (gained the last few years) to the Pentax prime experience (not you Prime) and hopefully they'll then spend some real $ on some Limited's.

Folks keep in mind this forum includes ALL Pentax DSLR's. Pro user's to beginners. There is a wide range of users with a wide range of needs. A relatively fast inexpensive, plastic general purpose prime will fill the needs of many average users. It might be insulting to some advanced users but you old fellas aren't exactly a growth industry for Pentax.

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Well said seeblue. I'm one of the "old fellas" you talk about and I agree with you. Heck, I might even buy one of those new plastic mount lenses myself. I think it will sell well as soon as the street prices come about.

Cheers.

Ron

--
Ron - 'We don't have time to go take pics this afternoon Carl.'
Carl - 'What do you mean? It will only take 1/1000s.'

'Keep your eyes looking forward. However, glance back now and then to see where you've come from. It will put a smile on your face.' ~ brandrx

Fast lenses are certainly not as needed as when we were shooting slow slide films (my fast film was Provia 100 pushed to ISO 200), funny

ISO 200 is considered low ISO now... what the heck!

I won't buy the DA35mm/2.4 because i have the DA35mm Macro Ltd...unless it's a marvelous lens !
--
Leopold
Pentax forever
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leopold said:

Fast lenses are certainly not as needed as when we were shooting slow slide films (my fast film was Provia 100 pushed to ISO 200), funny

ISO 200 is considered low ISO now... what the heck!

I won't buy the DA35mm/2.4 because i have the DA35mm Macro Ltd...unless it's a marvelous lens !

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Hi Leopold. Well, I have the FA*31/1.8 and the DA 35/2.8 Macro but I might just buy the DA 35/2.4 just because I can, and perhaps there will be some days I don't want to even take my more expensive lenses with me. My K2000 + 18-55mm + 50-200mm + DA 35/2.4 sure would make a lightweight kit.

Cheers.

Ron

--
Ron - 'We don't have time to go take pics this afternoon Carl.'
Carl - 'What do you mean? It will only take 1/1000s.'

'Keep your eyes looking forward. However, glance back now and then to see where you've come from. It will put a smile on your face.' ~ brandrx

I'm not taking a stand here, I just want to know

Is godfrog correct when he writes this in an earlier post in this thread about wide apertures and viewfinder clarity.

"Edit: as for the viewfinder, a F1.4 is no brighter in the viewfinder than a F2.8 prime or zoom. Try it yourself using the DOF preview, stopping down your F1.4 lens to F2.8. Fast lenses do not autofocus better either, since the AF system is only working with light corresponding to about F4.5-5.6 or so. Try finding a situation where your kit lens (at a focal length where the lens is F4.5 or faster) will not lock focus but your F1.4 prime will."

I've read basically the same point elsewhere as well ---Is the notion of improved viewfinder clarity with wider lenses something that was true for film cameras but is no longer true for DSLRs, or is it just one of those things that feels true but isn't, or has it always been and still is true? Inquiring minds and all that.....

.

you are right, Pentax should have made a 35mm F3.5 instead since speed doesnt matter at all.

And they should drop the metal mount too ! Plastic is cheaper and lighter !

IZO100 said:

you are right, Pentax should have made a 35mm F3.5 instead since speed doesnt matter at all.

And they should drop the metal mount too ! Plastic is cheaper and lighter !

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nope ! 35/5.6 as at 5.6 their AF is still guaranteed to work... but imagine the savings on plastic and paint ? and at that aperture they might just hit the world record in resolution wide open among all primes.

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geezer52 said:

geezer52 wrote:

I've seen several threads lately with complaints about how slow a 2.8 or 2.4 lens is and I'm not sure I understand why.

But it seems to me, the only valid argument for lenses wider than about 2.0 is greater control over depth-of-field. The lens speed itself seems to be a non-issue.

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Because you have tried it, so you know that any f-stop measure is a very subjective measure of quality, because photography is a subjective art anyway. Especially today when digital moves lightyears ahead of film-age rules. Only those who complain by demanding at the same time objectively comparable specs are those who take comparisons and numbers as their raison d'être, not photographic results.

See, new DA35/2.4, almost everyone has started a point Pentax is doomed because

• it's not f/1.4
• it's $20 more expensive than N, S or C's offerings

And that is such a predictable rant: people will whinge about whatever they get, 'cheap' or 'expensive', metal or plastic, f-this or f-that -- no matter what -- because they will under all circumstances compare Pentax with C, N and S and expect Pentax to be same as them. That's why they must show their protest: to justify their inability to imagine, or draw conclusions form the experience because they have none.

By flattening the world of interesting distinctiveness (and even all of those real world values that work in your favour and are not find in a competing manufacturer's lineup) to the plateau of absolute sameness is the only way for that kind of people to 'evaluate' photography equipment, draw any conclusion, or to re-assure themselves in their investment. For they're incapable of a photographic proof or insight, or of a clear "economy plus photography" equation evaluation that may help them understand what's really the case here.

I agree most of my low light shooting is with the DA*50-135 f2.8 and anything I could gain from a faster prime would be lost because of the need to step down. And any short fall can be compensated for by bumping up the ISO.

I shot these this week at a wedding (There was a hired pro so I shot for my own satisfaction) I used my Canon QL17 with 200 ASA fim. I would have loved to be able to stop down for some of the shots but I was shooting at 1/30 sec already. My point is that lens speed isn't the be all end all anymore now that we can control ISO.





For the FF army notice how shallow the DOF is on the second shot, I wish I could have stopped down or had a smaller sensor

A good body mount cover! It can even take a picture before a better lens can be used.

You can get away with a slower lens due to better resolution, high ISO in today's cameras, blah blah, but there can be no substitute for situations when more light would be advantageous, for flexibility, for really dim conditions, for DOF control, for image quality or just for show. All those expensive fast lenses are not made because manufacturers and photograhers are dumber than those who buy these expenive slow limited though tiny lenses.

snostorm said:

Everyone always seems to only consider DOF control as the only real advantage of fast lenses, but my main concern in many situations is whether the AF system has enough light to work with, as my eyes aren't good enough to MF in a dimly lit situation.
Scott

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EXACTLY!! Thank you, Scott.

Perhaps, some day, Lazik eye adjustment surgery will be able to help our older eyes.
Until then, I will use faster glass.
SK

awaldram said:

djezraj said:

Franka T.L. said:

As good as high ISO on DSLR are, they still cannot match using a fast lens on low ISO .. the fast speed , as stated, allow better control over DOF, but mostly it allow a lot more exposure options in various conditions.

It could mean able to freeze action on a dull day when still on low ISO, or ISO 400 RAW + JPEG capture ( so you stay within bound of not needing the NR ) instead of needing to push it to 3200 ISO .. and then some ....

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Sorry I cant really agree with this on the basis that freezing action is usually done stopped down in order to increase depth of field hence keeping more objects in focus.

While you may have the technique down, many will find freezing action at F1.8 or lower un necessarily challenging as only a sliver will be in focus.

If you check out most sports or action photography (which is largely nikon or Canon) many shots are stopped down and taken at high ISO in order to freeze the subject. Most pro's will expose correctly and add NR later to get the perfect print.

However you are welcome to prove me wrong I would love to see these shots. i am simply repeating what I have learned.

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My pleasure

awaldram said:

For Example my FA50 is a great lens stopped down but wide open the contrast suffers and the depth of field is razor thin. So its good for many things but perhaps not action at F1.4

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Well played friend well played! This is a stunning shot of course taken with a lens that totally outclasses what I was considering (cheaper primes) i dont think my FA50 could pull that off and if you can show me that then i will be even more impressed.

I still feel Pentax have probably chosen their compromise well by their own terms but they may pay for it. I personally wont hold it against them

by the way your shot really just makes me want to spend more money on that DA*55 1.4 and go swimming

Cheers
Roger
tp: www.flickr.com/photos/7925236@N02/

awaldram said:

godfrog said:

For subject isolation (short depth of field). For shooting in low light, often needing somewhat high shutter speeds to capture movement (I dont shoot still life photography in dark alleys...).

Fast lenses stopped down to F4-F5.6 are often some of the sharpest lenses around.

Sure, if you base all your shooting around what could be done in the film era, and do not care about subject isolation, then a K-x with the kit lens has all the performance you will ever need.

Edit: as for the viewfinder, a F1.4 is no brighter in the viewfinder than a F2.8 prime or zoom. Try it yourself using the DOF preview, stopping down your F1.4 lens to F2.8.

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I thought you were going to nail both mis conceptions here...you got the first one f2.8 is max viewfinder brightness but then fell flat on your but.

awaldram said:

Fast lenses do not autofocus better either,

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Yes they do, the f50 1.4 is an exception because of the drop in contrast wide open.
So the correct statement is decent fast lens focus better.

awaldram said:

since the AF system is only working with light corresponding to about F4.5-5.6 or so.

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That is top end not bottom thats why AF fails when you go much behond f5.6 i.e an f8 lens will not auto focus.

awaldram said:

Try finding a situation where your kit lens (at a focal length where the lens is F4.5 or faster) will not lock focus but your F1.4 prime will.

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My DA*55 will focus at 1-2 ev less than than the kit for any target.

Though my f2.8 zooms more so as they maintain contrast

AF performance is a mixture of light level and contrast of the lens, having special sensor for wider than f2.8 is a Canon Kludge due to theirAF inaccuracy.

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No.

Do you know how phase detect works? The following PDF has a pretty good introduction:
http://dougkerr.net/Pumpkin/articles/Split_Prism.pdf

The AF system simply does not see light from the periphery of the lens. A F0.7 lens does not give any more light to the AF system than a F4 lens, since the phase detect system is comparing two light beams from near the center of the lens.

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Note that I do agree with you ... but freezing action is not always sport, and I do note that its only really needed foir dull days / low light when able to capture the shoot is more important than caring for the optimal DOF.

And I must re-iterate . its not the DOF, its not the action freezing ( that's a side product of being fast and thus allow fast shutter ) its the ability to allow more exposure options. In fact DOF is only much a issue when you goto long focal / close subject distance. A simple say FA 50mm as yours when used to shoot subject say some 5m away would had ample amount of DOF even on a DSLR when used at say f/2.4 ( which is already 1.5 stop stopped down from the f/1.4, and yet still 2.5 stop wider than typical kit zoom ( which wide open at 50mm usually around 5.6 ) That would also mean the ability to maintain ISO 400 and not pushing the SR to extreme ( which usually also mean more leeway with the shoot )

One thing about freezing action. its not a sport thing. Its ctually very much anything that moves, and that;'s a lot in casual / day to day / nature shoots ( picture a field of wild flower in light breeze, or the cat just running around the house )

I believe in many ways people had simply got it too focused about the speed used for shallow DOF, and forget that while that might be the prime reason. its not the initital reason why people need fast lens ( as in the original post, ist to allow shoots )

--

• Franka -

Depends on the focus screen, most focus screens are claibrated to deliver a brightness and DoF of about f2.8, if you wanted to manual focus lenses that were faster, you needed to by a faster screen, the problem with the faster screens is that they were almost unusable with lenses that were f4 or slower.

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djezraj said:

awaldram said:

djezraj said:

For Example my FA50 is a great lens stopped down but wide open the contrast suffers and the depth of field is razor thin. So its good for many things but perhaps not action at F1.4

Click to expand...

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Well played friend well played! This is a stunning shot of course taken with a lens that totally outclasses what I was considering (cheaper primes) i dont think my FA50 could pull that off and if you can show me that then i will be even more impressed.

I still feel Pentax have probably chosen their compromise well by their own terms but they may pay for it. I personally wont hold it against them

by the way your shot really just makes me want to spend more money on that DA*55 1.4 and go swimming

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Thanks Roger just having a bit of fun.

Actually I agree with everything you said, and think we over estimate speed pretty much as we over emphasise how good the limiteds are.

In reality high speed and hi iq glass is only marginally better than the kit lens and in the hands of many (me included) a total waste.

djezraj said:

Cheers
Roger
tp: www.flickr.com/photos/7925236@N02/

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godfrog said:

awaldram said:

godfrog said:

For subject isolation (short depth of field). For shooting in low light, often needing somewhat high shutter speeds to capture movement (I dont shoot still life photography in dark alleys...).

Fast lenses stopped down to F4-F5.6 are often some of the sharpest lenses around.

Sure, if you base all your shooting around what could be done in the film era, and do not care about subject isolation, then a K-x with the kit lens has all the performance you will ever need.

Edit: as for the viewfinder, a F1.4 is no brighter in the viewfinder than a F2.8 prime or zoom. Try it yourself using the DOF preview, stopping down your F1.4 lens to F2.8.

Click to expand...

I thought you were going to nail both mis conceptions here...you got the first one f2.8 is max viewfinder brightness but then fell flat on your but.

godfrog said:

Fast lenses do not autofocus better either,

Click to expand...

Yes they do, the f50 1.4 is an exception because of the drop in contrast wide open.
So the correct statement is decent fast lens focus better.

godfrog said:

since the AF system is only working with light corresponding to about F4.5-5.6 or so.

Click to expand...

That is top end not bottom thats why AF fails when you go much behond f5.6 i.e an f8 lens will not auto focus.

godfrog said:

Try finding a situation where your kit lens (at a focal length where the lens is F4.5 or faster) will not lock focus but your F1.4 prime will.

Click to expand...

My DA*55 will focus at 1-2 ev less than than the kit for any target.

Though my f2.8 zooms more so as they maintain contrast

AF performance is a mixture of light level and contrast of the lens, having special sensor for wider than f2.8 is a Canon Kludge due to theirAF inaccuracy.

Click to expand...

No.

Do you know how phase detect works? The following PDF has a pretty good introduction:
http://dougkerr.net/Pumpkin/articles/Split_Prism.pdf

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There is nothing in this document that refutes my claim.. (extra sensors are a Canon Kludge as there original design used too wide baseline to support accurate AF on
Hence why the document refers explicitly to Canon SLR's

only if the two “virtual AF apertures” are spaced closer than the full diameter of the exit pupil will AF be impaired for wider aperture lens.

I don't have the data for Pentax Baselines but statistical evidence shows it is considerably longer than Canons f5.6.

Unlike Canon , Pentax has never had an issue with focus accuracy for wide lens ergo the baselines for Pentax AF system are long enough to support wide aperture lens whilst being accurate enough not to be compromised with f5.6 and in the case of the super zooms f6.3.

I think you being very unfair trying imply Pentax need to resolve Canon design deficiencies rather than celebrate Pentax got it right first time.

godfrog said:

The AF system simply does not see light from the periphery of the lens.

A F0.7 lens does not give any more light to the AF system than a F4 lens, since the phase detect system is comparing two light beams from near the center of the lens.

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What????

I never said it did......oh I see "light level" I mean real light level not aperture controlled light level.

i.e how light the environment is + the contrast of the target and lens affect AF performance.

godfrog said:

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Couple of examples of stopping action wide open, both 2.0 on FA50.

ISO3200 on K7. Anything slower would mean I would have to slow down the shutter and get more blur, at the same time, the subject is sharp, but isolated.

Edit: I know this is an extreme situation with bad light, shooting through a net and really fast movement, but it is something I like shooting, so anything slower than 2.0 is just too slow (for this). Others may not need the speed, but I for one do.





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skeys said:

snostorm said:

Everyone always seems to only consider DOF control as the only real advantage of fast lenses, but my main concern in many situations is whether the AF system has enough light to work with, as my eyes aren't good enough to MF in a dimly lit situation.
Scott

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EXACTLY!! Thank you, Scott.

Perhaps, some day, Lazik eye adjustment surgery will be able to help our older eyes.
Until then, I will use faster glass.
SK

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But too thin a DOF makes AF accuracy worse. To improve AF you need to stop down.