Opinions on focus comparison please

I notice the EXIFs for these shots show a six second exposure. You do have to be careful when using a tripod for long exposures - just the fact of using the tripod doesn't guarantee a stable image and any lack of stability whatsoever will adversely affect the sharpness, which may be what we're seeing in these images. There are several precautions you can take to make sure the setup is as stable as possible when using a tripod and cable release:

- Use a good quality tripod! Cheap tripods are more prone to unwanted movement.

- Weight your tripod if possible, especially if it is a fairly light tripod. Some tripods have a ballast hook for this purpose.

- Ensure all locking rings, knobs etc. on the tripod and head are fully tightened.

- Remember to turn off SR (or IS in the case of Canon lenses which have it).

- Use mirror lockup mode if possible.

- Stand completely still for several seconds before taking the shot and remain still until the exposure is over. Any movement on your part can be transmitted through the ground to your tripod causing vibrations so give any residual vibrations time to die down first.

- Be aware of environmental factors which may cause movement in your tripod such as wind,heavy traffic or even just pedestrians passing close by (this can cause vibrations in the ground). Of course depending on the situation there may be nothing you can do about such factors.

Well spotted Mark, that is more than likely the cause of the poor Pentax image.

Doubtful. Although Mark's advice is very good.

Both images were taken under exact same conditions (same tripod, same 6 sec exposure), and confirm the OP's general observation that his K5 images are soft.

The only thing that may have caused more softness in the Pentax is the SR, which I have already brought up. This needs to be turned off. But I don't think this is the cause of this amount of blur, especially when the softness is borne out by the OP's observations under normal conditions.

thanks guys - I'll use jpeg for my next round of tests - thanks a lot for all your help

I'll perhaps do a mix of more "technical" type set ups as John suggests and some more "real world" shots.

thanks again to all

Yes next time I'll ensure SR/IS is switched off and maybe look at mirror lock up or indeed use in a brighter environment so such a long exposure is not needed.

thanks again all

'Real world' shots are invariably useless for testing focus accuracy. It's virtually impossible to compare like with like that way.

The first thing to do, as JR said, is use a test chart at 45 degrees to ensure that the camera is actually focused where it thinks it's focused. This requires care, to ensure that the camera is pointed accurately at the centre of the chart, and is straight on to the chart. Otherwise, all conclusions are invalid.

Use a delayed mode release so that SR is guaranteed off. Ideally, have plenty of light so that all the worry about camera shake is excluded.

Only then, once you've ensured the AF is adjusted properly for the lens, is it worth proceeding to look at focus over the field of view, ideally at different apertures.

You need a subject with fine detail (brickwork, or a newspaper taped flat to a wall for example), and again, you need to ensure that the camera/lens axis is accurately perpendicular to the subject. Otherwise, if it's sharp at one side but not the other, you can't be sure it's not because the camera's at an angle.

Controlled conditions such as these are hard enough to test reliably, without introducing all the other variables you get with 'real world' shots.

True, but there is no way of telling whether external factors which could cause movement in the tripod were the same for both shots. Hence the last two points I mentioned. A gust of wind or operator movement at the wrong time can have its effect.

One of our former members, Richard Day, suggested this method;

"I don't like test charts either, they have always given me misleading results.

I setup using a near infinity subject like a car number plate and working in 5 unit steps from -10 to +10, I review the results and work around the best in 1 unit steps. I always recheck the AF from both directions to ascertain there is minimal backlash.

if doing it for a zoom lens I check at the wide, mid and long ends.

I then repeat the above for a mid distance and then a near distance (approx 2m).

I generally find that-the results are pretty close and adopt a nominal best setting, critical focus is fairly easy to see as the text usually becomes very dense and sharp. When OOF, the text tends to loose contrast and small halos appear around the edges. I usually review my results on the rear screen at xll setting (which is approx 1:1 on a K20D)

When I check my setting at macro distances, i.e. the closest focus point, using a test chart, I usually find that there is more DOF behind the focus point than in front (but the focus point is sharp), if I set for equal balance, I find critical focus on most objects at further distances become more OOF as you approach distance/infinity objects.

I’ve heard it stated by ex-Pentax engineers, that Pentax lenses are designed to perform in this manner as well as many others.

Anyway, it works for me! I find that most of my lenses require quite minimal adjustments, but it does make a difference.

As a footnote, I would never buy a camera now that didn't have individual AF micro adjustments."

If you search the forums you'll find this has come up for discussion/debate many times

Ok .. thought I would join in the fun.

Camera 1 = Pentax K-5 with Sigma MKI 70-200mm f2.8
Camera 2 = Canon 50D with Sigma MKI 70-200mm f2.8

Both shots taken wide open in darkish conditions, converted from raw with no PP done whatsoever. I turned off SR on the K-5, focus is on the cup face.

2 things I did notice ... the difference in auto WB is interesting and the Canon hit focus in one go whereas the Pentax went forward/back/forward/back a bit/back a bit more/forward (well you get the general idea) and probably took more than twice as long to get there. This could be due to the Canon version lens having HSM whereas the same version in Pentax never had it and is screw drive.

As for actual sharpness, I would say there is not much in it but the Canon has the edge.

Multi segment metering
ISO 1600
1/20th
f2.8

K-5

[IMG]http://img.photobucket.com/albums/v188/Sockpuppetuk/K-5.jpg[/IMG]

Crop

[IMG]http://img.photobucket.com/albums/v188/Sockpuppetuk/K-5Crop.jpg[/IMG]

50D

[IMG]http://img.photobucket.com/albums/v188/Sockpuppetuk/50D.jpg[/IMG]

Crop

[IMG]http://img.photobucket.com/albums/v188/Sockpuppetuk/50DCrop.jpg[/IMG]

Hmmm, looking at them now (the straw) they are posted I wonder if the K-5 lens isn't front focusing a little more than I have adjusted for.

I'd put it the other way round, myself.

And the Canon image is way noisier.

The 50D was released virtually the same time as the K20D so if you want to compare ISO performance you should be comparing it with that

The K5 looks like it's focused slightly behind the face of the cup. Are you sure it didn't try and focus on the straw? The 50D looks to me like it's somewhere slightly in front.

Sorry, meant back focusing .. not concentrating.

According to Photome they both hit exactly the same focus point but the K-5 spreads its points out further so it may be including more of the cup (straw) in its calculations.

Way back when I had serious focus problems with my old K10D I found Tim Jackson's focus test chart and method and I have been using it ever since for all my short/medium distance lenses, when newly purchased or when I get a new camera.

Most of of the time it usually reveals only a small adjustment is required but it has highlighted a couple of rouges in my collection. One of my two DA18-55MKII kit lenses is miles out and my DA18-250 produces wildly different results at 18mm, 50mm and 100mm.

It is probably not such a good method for longish lenses where I think Richard Day's method is probably a much better approach but I think you have got to start somewhere with something that is fairly simple and repeatable and I certainly recommend this method and chart. Just remember that it is best done in natural lighting.

It comes with a description of how to use it and although is aimed at the Nikon D50 works fine with Pentax DSLRs for me.

The chart and instructions can be found here.


Glyn