has focus always been this bad on SLRs and we just didn't notice?

[ElFenix]

seems every new SLR and lens comes out and has focus issues one way or other.

the way an SLR focuses, however, it has a very short rangefinder base which makes it nearly impossible to precisely and accurately focus anything below an 85 f/1.8.

the dof for wide angle, small aperture lenses used in landscape situations largely masks the issue. but the razor thin dof on lenses like 50 f/1.4 means that missing focus by even a few mm causes a very poor result

but that didn't matter when the most many photos would be blown up was 8x10. larger than 8x10 and commercial applications like magazines were largely the domain of medium and large formats.

but now everyone can easily view their photos at 100% on a computer monitor, which exacerbates the short base length that SLRs have (just think about how much bigger each pixel is on your monitor than on a 10 MP APS sensor).

 

[soydios]

Back in the olden days before autofocus, the viewfinders were big and bright, because ALL the light went to the viewfinder; none was diverted down at the main mirror towards the autofocus unit. Furthermore, those cameras had split-prism focusing screens, which make it instantly apparent if your focus is good or not.
If critical focus is important to you, then I would recommend a split-prism focusing screen. Otherwise, I'll say that I have been quite pleased with the AF performance of my D50, especially in light of the low price; I consider it a bargain.

 

[randomlinh]

I think it's more of a problem for pixel peepers. I am guilty of it too sometimes when testing a lens I buy to ensure it's not completely off. I have been trying to just take steps back to "well, I need to work more on my actual picture taking skills first" because in actual use of my pictures, the focus/sharpness is actually fine.

But yeah, razor thin DOF is just difficult to work with in general.. and I tend to MF when I can with it.

 

[Anubis]

the same issues plagued film in realy shallow DOF settings, IDK if its a camera fault but when your subject is only 8mm long and you are off by 2mm it makes a huge difference

 

[ElFenix]

Originally posted by: soydios
Back in the olden days before autofocus, the viewfinders were big and bright, because ALL the light went to the viewfinder; none was diverted down at the main mirror towards the autofocus unit. Furthermore, those cameras had split-prism focusing screens, which make it instantly apparent if your focus is good or not.
If critical focus is important to you, then I would recommend a split-prism focusing screen. Otherwise, I'll say that I have been quite pleased with the AF performance of my D50, especially in light of the low price; I consider it a bargain.

the viewfinders for most film cameras had lower magnification than most of today's SLRs, so focusing accurately would actually be more difficult with the lower magnification (demagnification really) for any given lens, though the brightness would certainly help.

base length is absolute aperture size * magnification. many dslrs, to combat the fact that the image sensor is small, have very high magnification. film cameras didn't, both because it'd require a larger prism and because lower magnification should make the finder brighter.

 

[ivan2]

can i get the split prism focusing screen for 30D or is that a Nikon or 40D thing?

 

[Anubis]

Originally posted by: ivan2
can i get the split prism focusing screen for 30D or is that a Nikon or 40D thing?

its an oldschool thing, most cameras dont have them, and IIRC you cant change out the focusing prisms any cams other then top of the line pro ones

 

[ElFenix]

Originally posted by: ivan2
can i get the split prism focusing screen for 30D or is that a Nikon or 40D thing?

i think the katz eye is a split prism focusing screen. it'll help, but it's accuracy still depends on the absolute aperture of the lens it's looking through. if your dof isn't big enough to cover the margin of error of that short base range, it won't help that much.

 

[soydios]

Originally posted by: ElFenix

Originally posted by: ivan2
can i get the split prism focusing screen for 30D or is that a Nikon or 40D thing?

i think the katz eye is a split prism focusing screen. it'll help, but it's accuracy still depends on the absolute aperture of the lens it's looking through. if your dof isn't big enough to cover the margin of error of that short base range, it won't help that much.

and so we go to 1:1 pixel-for-pixel live view from the main sensor.

I dunno what cameras you've had, but I'd think that most any DSLR system on the market today can deliver 100%-sharp shots under most conditions, even less-than-ideal ones. And if you know the ins-and-outs of your equipment, you can get sharp photos in rather challenging conditions, too. Most cameras are far more capable than their operators (myself included).

 

[virtuamike]

Originally posted by: ElFenix

Originally posted by: ivan2
can i get the split prism focusing screen for 30D or is that a Nikon or 40D thing?

i think the katz eye is a split prism focusing screen. it'll help, but it's accuracy still depends on the absolute aperture of the lens it's looking through. if your dof isn't big enough to cover the margin of error of that short base range, it won't help that much.

I had one in my D2x and I can't recommend it. Marginally easier to see if things were in focus but much harder overall to work due to viewfinder being darker. In bright situations it's not a huge issue, but if you're using fast primes in low light situations then it really doesn't help.

 

[randomlinh]

Originally posted by: virtuamike

Originally posted by: ElFenix

Originally posted by: ivan2
can i get the split prism focusing screen for 30D or is that a Nikon or 40D thing?

i think the katz eye is a split prism focusing screen. it'll help, but it's accuracy still depends on the absolute aperture of the lens it's looking through. if your dof isn't big enough to cover the margin of error of that short base range, it won't help that much.

I had one in my D2x and I can't recommend it. Marginally easier to see if things were in focus but much harder overall to work due to viewfinder being darker. In bright situations it's not a huge issue, but if you're using fast primes in low light situations then it really doesn't help.

damn, it makes it darker? Did you have the "optibrite" or whatever coating they offer? I am tempting to get this so it will make focusing w/ my Super Tak easier, but $100 on a 4yr old camera kinda bites, heh.

 

[corkyg]

Take a look at what Haoda Fu can offer. He custom makes very nice split prism screens for most DSLRs.

Haoda

 

[Zenmervolt]

SLR's have a rangefinder base? That's news to me. I always thought SLR focusing was through the lens only (single point of light entry). Where's the second window letting light in for the rangefinder system?

EDIT: This isn't meant as a challenge, I'm genuinely confused.

ZV

 

[ElFenix]

Originally posted by: Zenmervolt
SLR's have a rangefinder base? That's news to me. I always thought SLR focusing was through the lens only (single point of light entry). Where's the second window letting light in for the rangefinder system?

EDIT: This isn't meant as a challenge, I'm genuinely confused.

ZV

from wikipedia on phase detect autofocus:

Phase detection is achieved by dividing the incoming light into pairs of images and comparing them. SIR TTL passive phase detection (secondary image registration, through the lens) is often used in film and digital SLR cameras. The system uses a beam splitter (implemented as a small semi-transparent area of the main reflex mirror, coupled with a small secondary mirror) to direct light to an AF sensor at the bottom of the camera. Two optical prisms capture the light rays coming from the opposite sides of the lens and divert it to the AF sensor, creating a simple rangefinder with a base identical to the lens' diameter. The two images are then analysed for similar light intensity patterns (peaks and valleys) and the phase difference is calculated in order to find if the object is in front focus or back focus position. This instantly gives the exact direction of focusing and amount of focus ring's movement. Although AF sensor is typically a one-dimensional photosensitive strip (only a few pixels high and a few dozen wide), some modern cameras (Canon EOS-1D, Nikon D2X) feature Area SIR sensors that are rectangular so as to provide two-dimensional intensity patterns. Cross-type (CT) focus points have a pair of sensors oriented at 90° to one another, although one sensor typically requires a larger aperture to operate than the other. Some cameras (Canon EOS-1D, Canon EOS 30D/40D) ) also have a few 'high precision' focus points with additional set of prisms and sensors; they are only active with 'fast lenses' of certain focal ratio. Extended precision comes from the increased diameter of such lenses, so the base of the 'range finder' can be wider.

and if you have a split image rangefinder built into your camera, then there it is. i found some approximations for SLR base lengths (changes depending on the magnification and the angle of split in the prism):

f=21mm --> b'=1.63mm
f=28mm --> b'=2.78mm
f=50mm --> b'=9.82mm
f=90mm --> b'=29.20mm
f=135mm --> b'=65.70mm

if those numbers are anywhere near correct then split prisms are inadequate for focusing faster than 50 f/2.8 or 28 f/4. however, it's perfectly capable of focusing a 135 f/2.8.

 

[Zenmervolt]

Originally posted by: ElFenix

Originally posted by: Zenmervolt
SLR's have a rangefinder base? That's news to me. I always thought SLR focusing was through the lens only (single point of light entry). Where's the second window letting light in for the rangefinder system?

EDIT: This isn't meant as a challenge, I'm genuinely confused.

ZV

from wikipedia on phase detect autofocus:

Phase detection is achieved by dividing the incoming light into pairs of images and comparing them. SIR TTL passive phase detection (secondary image registration, through the lens) is often used in film and digital SLR cameras. The system uses a beam splitter (implemented as a small semi-transparent area of the main reflex mirror, coupled with a small secondary mirror) to direct light to an AF sensor at the bottom of the camera. Two optical prisms capture the light rays coming from the opposite sides of the lens and divert it to the AF sensor, creating a simple rangefinder with a base identical to the lens' diameter. The two images are then analysed for similar light intensity patterns (peaks and valleys) and the phase difference is calculated in order to find if the object is in front focus or back focus position. This instantly gives the exact direction of focusing and amount of focus ring's movement. Although AF sensor is typically a one-dimensional photosensitive strip (only a few pixels high and a few dozen wide), some modern cameras (Canon EOS-1D, Nikon D2X) feature Area SIR sensors that are rectangular so as to provide two-dimensional intensity patterns. Cross-type (CT) focus points have a pair of sensors oriented at 90° to one another, although one sensor typically requires a larger aperture to operate than the other. Some cameras (Canon EOS-1D, Canon EOS 30D/40D) ) also have a few 'high precision' focus points with additional set of prisms and sensors; they are only active with 'fast lenses' of certain focal ratio. Extended precision comes from the increased diameter of such lenses, so the base of the 'range finder' can be wider.

and if you have a split image rangefinder built into your camera, then there it is. i found some approximations for SLR base lengths (changes depending on the magnification and the angle of split in the prism):

f=21mm --> b'=1.63mm
f=28mm --> b'=2.78mm
f=50mm --> b'=9.82mm
f=90mm --> b'=29.20mm
f=135mm --> b'=65.70mm

if those numbers are anywhere near correct then split prisms are inadequate for focusing faster than 50 f/2.8 or 28 f/4. however, it's perfectly capable of focusing a 135 f/2.8.

Very cool, I did not know that.

I do know that I never had a problem focusing a 50mm f/1.7 lens with my old film SLRs that used split-prism finders, but that may have been partially due to the help of the ground glass and microprism collar rather than just the split image.

ZV

 

[ElFenix]

Originally posted by: Zenmervolt

Very cool, I did not know that.

I do know that I never had a problem focusing a 50mm f/1.7 lens with my old film SLRs that used split-prism finders, but that may have been partially due to the help of the ground glass and microprism collar rather than just the split image.

ZV

how big did you blow up the images?