[Brangdon]

I wonder if it is still necessary to have fast lens, i.e. f/2 or f/2.8.

Interesting that you consider f/2.8 to be fast. I tend to think of fast as being below f/2. The new 35mm/2.4 was criticised by some for not being fast.

My feeling is that f/2.8 is very useful, and when going faster than f/2 you get diminishing returns. The depth of field gets so thin it is harder to focus. Obviously some people like and use it, and when you ask online you'll find such people, but it's less important now than it was five years ago.

For example, I recently bought the DA 35mm Ltd macro, and it didn't bother me at all it was only f/2.8. There are faster lenses around at or near that focal length, but the benefits of a DA Ltd outweigh the benefits of speed, in my opinion.

[abacus07]

dumb question - how does the DOF change with crop factor?

Obviously the focal length changes and that is one part of the equation. Yes, I'm just too lazy to look this up.

[Winder]

I think if Pentax is going to stay with APS-C exclusively then they actually need to move to faster glass. They need to do what Olympus did and design their 50-135 as an f/2.0. The only way to match 135mm for depth of field is to move to faster glass. Faster glass will also help to offset the speed disadvantage of the higher pixel density sensors.

[Ash]

dumb question - how does the DOF change with crop factor?

Obviously the focal length changes and that is one part of the equation. Yes, I'm just too lazy to look this up.

Already answered in this thread:
https://www.pentaxforums.com/forums/1299779-post24.html

[Opethian]

This thread has been very informative. I was justifying an expense I'll be having sometime in 2011, with a good 70-200 2.8 purchase (still undecided between a Tamron or a Sigma) or just go for the Tamron 90mm 2.8 Macro.

What I would like to know though is, will the "new" sensors in the latest K series be justifiable for you to upgrade from say, a K10D?

[Kerrowdown]

By the nature of what I do, for me it's a big yes, but I appreciate it's a fairly specialist requirement that I have and therefore does not apply to maybe most folk.

Much of my work is concert photography, in near darkness or very low light, requiring the use of at least ISO 1600 or above and even then, most shots have to be taken in the range of F1.2 to F2.8 to get even half decent shutter speeds.

Don't get me wrong, where the situation allows at some of well lit bigger venues, F2.8 and above is just joy to use.

[Winder]

By the nature of what I do, for me it's a big yes, but I appreciate it's a fairly specialist requirement that I have and therefore does not apply to maybe most folk.

Much of my work is concert photography, in near darkness or very low light, requiring the use of at least ISO 1600 or above and even then, most shots have to be taken in the range of F1.2 to F2.8 to get even half decent shutter speeds.

Don't get me wrong, where the situation allows at some of well lit bigger venues, F2.8 and above is just joy to use.

I'm still using my Canon 5D for concerts and events. Pentax needs to improve the low-light AF accuracy and speed. They need faster glass and some longer glass.
135mm f/1.8 and an 85mm f/1.4 would really help the Pentax lens line up.

[opiet70]

I've been looking at the Sigma 300 2.8 coupled with their 2x TC. According to their documentation, anything over f 5.6 doesn't autofocus well (I've seen varying accounts of this...). That would be my main desire for a 'fast' lens - the ability to strap a TC onto it and still capture good photos.

But, the other thing comes down to my individual setup and wants.

The K10D sucks at high ISOs - so that seriously limits 1/3 of my options (aperture, ISO, shutter speed) when shooting wildlife. I do plan to upgrade to the K-5 at some point in time which will help. The other piece is a lot of the shots of what I want are best taken early or late in the day. For example, I can go back to Nebraska and take pictures of dozens of whitetail deer all in one spot at sunrise or dusk. Because it's darker (and I'd like action shots - not grazing shots), I have to either slow shutter speed (bad for action) decrease aperture, or increase ISO. As such, the faster lens I have the better because it provides me with more options.

The question I have to ask myself is simple: Is the extra cash (~$1800 between the DA 300 f4 and Sigma 300 f2.8) worth what I can do with it or do I pursue other options that may give me the same results? I debated this a month or two ago with another forum member and they made some good points. The question comes down to how much is the extra stop of flexibility worth to me?

[northcoastgreg]

What I would like to know though is, will the "new" sensors in the latest K series be justifiable for you to upgrade from say, a K10D?

Yes, the new sensors are worth the upgrade, whether one likes to shoot in low light or not. They are a big help in wildlife or sports photography. Right now I can't take any pictures of wildlife moving on my K200D, because I can't get a fast enough shutter speed at ISO 400 (and it's really not worth going higher on that camera). The dynamic range of the K-5 makes it a compelling camera for landscape as well.

Obviously some people like and use [f-stops lower than 2.0], and when you ask online you'll find such people, but it's less important now than it was five years ago.

Maybe so. I haven't noticed that myself. It seems that interest in and appreciation of fast lenses has grown in the digital age. The instant feedback of digital has greatly improved photographic skill and technique, and photographers are now trying to explore every avenue possible to take their artistry up that additional notch. Using fast lens to experiment with razor thin DOF is just another those avenues photographers are avid to explore. In the days of film, it was often too costly to experiment (developing film could be expensive); plus the time it took to get the results back took half the fun out of it. You didn't want to waste shots, because that meant wasting film. And so a lot of photographers just played it safe.

[Frogfish]

Depth of field control still remains a need for artistic shots, and this can only be truely achieved with fast lenses, and this is becoming much more apparent with ASP-C format cameras, where the depth of field with wide angle lenses (equal FOV to film or full frame) is higher, This is what is pushing fast wide and ultra wide lenses today.

Or good PP skills

[Pentaxor]

Or good PP skills

you got be more than that.

[Frogfish]

I've been looking at the Sigma 300 2.8 coupled with their 2x TC. According to their documentation, anything over f 5.6 doesn't autofocus well (I've seen varying accounts of this...). That would be my main desire for a 'fast' lens - the ability to strap a TC onto it and still capture good photos.

But, the other thing comes down to my individual setup and wants.

The K10D sucks at high ISOs - so that seriously limits 1/3 of my options (aperture, ISO, shutter speed) when shooting wildlife. I do plan to upgrade to the K-5 at some point in time which will help. The other piece is a lot of the shots of what I want are best taken early or late in the day. For example, I can go back to Nebraska and take pictures of dozens of whitetail deer all in one spot at sunrise or dusk. Because it's darker (and I'd like action shots - not grazing shots), I have to either slow shutter speed (bad for action) decrease aperture, or increase ISO. As such, the faster lens I have the better because it provides me with more options.

The question I have to ask myself is simple: Is the extra cash (~$1800 between the DA 300 f4 and Sigma 300 f2.8) worth what I can do with it or do I pursue other options that may give me the same results? I debated this a month or two ago with another forum member and they made some good points. The question comes down to how much is the extra stop of flexibility worth to me?

That difference in price buys you a new K5 which would be a massive improvement over your K10D but of course effectively improve all of your lenses too. I don't have it but from the shots I've seen online the DA*300 is a stellar piece of equipment and paired with a K5 should beat the pants off of a Sigma 300 and a K10D.

Other than that I can only foresee a change to a 7D (not a 5DII FF as then you would then need still longer glass) as a way to help you out of your predicament (and once you've added glass you are going to be well over your $1,800 anyway).

[RioRico]

AFAIK the issues re: fast glass are 1) low-light and/or fast shutter, 2) thin DOF, 3) weight and cost. My FA50/1.4 is light and was only US$200 new. My next fastest lenses, a Sears-Tomioka 55/1.4 (US$3 recently), a Vivitar-Kiron 24/2 (US$135 recently) and a Nikkor 85/2 (US$9 a couple years ago) are rather bigger and heavier and manual. That Viv doesn't exactly produce thin DOF, but the others are quite good at surgically separating subjects from backgrounds.

I've mentioned before a DOF INDEX: divide the focal length by the aperture. The bigger the quotient, the thinner the DOF. My thinnest DOF comes from a 127/1.8 projector lens (mounted on bellows) that cost me ONE BUCK a few weeks ago. For portraits, focus and aperture automation are optional, so bellows lenses are fine. Edge-sharpness isn't really mandatory either. We can drastically reduce DOF of any lens by strapping-on a +1 dioptre closeup adapter, the kind that comes in a set of three for US$10.

That leaves low-light, where we can either boost the ISO and accept the noise, or buy faster glass. And/or use a tripod, flash, IR, various other tricks. Whaddya mean, IR? Well, I'm not sure how a dSLR with its IR-blocking hot.filter replaced with optical glass will function; but on my Sony NightShot DSC-V1, when I push the lever to flick the hot.filter out of the way, the camera's maximum ISO jumps from 800 to 2500. I can mount an IR light in the flash shoe and shoot in absolute darkness.

So, are fast lenses still necessary? Yes and no. Even with all available tricks, fast glass still has straightforward advantages. For birders and sports shooters, no lens will ever be fast or long enough. Fast shutters on unmodified cameras in dim light will always require fast glass. But DOF control can be cheaply achieved. Will a +1 dioptre on a 50/1.4 give thinner DOF than a 58/1.2 wide open? Somebody, loan me a 58/1.2 so I can test and see! (Hint: by DOF Index, a 135/2.5 beats that 58/1.2!)

[Rondec]

Of course, fast glass is still necessary, but I warrant that most people who shoot with fast glass stop down considerably as much as possible. I would far rather shoot at f4 than f2.8 on a cropped sensor and in general don't like portraits shot at f1.4 (although I really like Tod's second photo).

The other thing about fast lenses is that they are sharper when stopped down.

[Pentaxor]

AFAIK the issues re: fast glass are 1) low-light and/or fast shutter, 2) thin DOF, 3) weight and cost. My FA50/1.4 is light and was only US$200 new. My next fastest lenses, a Sears-Tomioka 55/1.4 (US$3 recently), a Vivitar-Kiron 24/2 (US$135 recently) and a Nikkor 85/2 (US$9 a couple years ago) are rather bigger and heavier and manual. That Viv doesn't exactly produce thin DOF, but the others are quite good at surgically separating subjects from backgrounds.

I've mentioned before a DOF INDEX: divide the focal length by the aperture. The bigger the quotient, the thinner the DOF. My thinnest DOF comes from a 127/1.8 projector lens (mounted on bellows) that cost me ONE BUCK a few weeks ago. For portraits, focus and aperture automation are optional, so bellows lenses are fine. Edge-sharpness isn't really mandatory either. We can drastically reduce DOF of any lens by strapping-on a +1 dioptre closeup adapter, the kind that comes in a set of three for US$10.

That leaves low-light, where we can either boost the ISO and accept the noise, or buy faster glass. And/or use a tripod, flash, IR, various other tricks. Whaddya mean, IR? Well, I'm not sure how a dSLR with its IR-blocking hot.filter replaced with optical glass will function; but on my Sony NightShot DSC-V1, when I push the lever to flick the hot.filter out of the way, the camera's maximum ISO jumps from 800 to 2500. I can mount an IR light in the flash shoe and shoot in absolute darkness.

So, are fast lenses still necessary? Yes and no. Even with all available tricks, fast glass still has straightforward advantages. For birders and sports shooters, no lens will ever be fast or long enough. Fast shutters on unmodified cameras in dim light will always require fast glass. But DOF control can be cheaply achieved. Will a +1 dioptre on a 50/1.4 give thinner DOF than a 58/1.2 wide open? Somebody, loan me a 58/1.2 so I can test and see! (Hint: by DOF Index, a 135/2.5 beats that 58/1.2!)

although you lose infinity with a diopter on, also increases whatever IQ degradation it will create (CA, distortion, loss of sharpness, vignetting, etc...).