Not the usual focus question.

Using the more or less current quality zoom lenses, which of these two methods of focusing is most accurate? 1. Zoom to the composition you like. Use the center focus point, and shoot. 2. Zoom to maximum to your (previous) focus point. Lock the focus. Zoom out to the composition you like. Use the center focus point, and shoot. Note, I am not considering other methods for now such as tripods, live view 10x magnification, etc. A more succinct question might have been: Can I change the zoom and not lose focus?

I am pretty sure most zooms are not true parfocal and do not hold there focus as you zoom in and out.

kbhartman said:

Using the more or less current quality zoom lenses, which of these two methods of focusing is most accurate? 1. Zoom to the composition you like. Use the center focus point, and shoot. 2. Zoom to maximum to your (previous) focus point. Lock the focus. Zoom out to the composition you like. Use the center focus point, and shoot. Note, I am not considering other methods for now such as tripods, live view 10x magnification, etc. A more succinct question might have been: Can I change the zoom and not lose focus?

Click to expand...

--
http://www.TheSBimage.com

kbhartman said:

Using the more or less current quality zoom lenses, which of these two methods of focusing is most accurate? 1. Zoom to the composition you like. Use the center focus point, and shoot. 2. Zoom to maximum to your (previous) focus point. Lock the focus. Zoom out to the composition you like. Use the center focus point, and shoot. Note, I am not considering other methods for now such as tripods, live view 10x magnification, etc. A more succinct question might have been: Can I change the zoom and not lose focus?

Click to expand...

Some variation of the first one is best. You might want to use other than the center focus point, you also might want to focus on something other than the center of your composition when using the center focus point, and then lock the focus and recompose to shoot.

Typically zoom lenses that are described as Variable Focal Length, as opposed to Zoom, will not maintain focus as the focal length is changed. Generally a VFL is less expensive to manufacture and can also be lighter, so they are not unheard of.

Regardless of that, if you use a zoom sometimes there can indeed be an advantage to focusing at maximum focal length, locking the focus, and then composing as desired. That is extremely commonly in video and cinematography, where you can rest assured that virtually every time a shot is zoomed in on that it was pre-focused at the longest focal length and then backed off to the shorter one for the actually shooting. The significance of focusing at a long focal length is that focus is more critical and has less depth of field, hence focus tracking error as the focal length is shortened will not likely be as significant as would happen if initial focus was at the shorter length.

Raymond Bradlau said:

I am pretty sure most zooms are not true parfocal and do not hold there focus as you zoom in and out.

Click to expand...

See these two articles for clarification:

http://en.wikipedia.org/wiki/Zoom_lens
http://en.wikipedia.org/wiki/Parfocal_lens

Generally if it is not described as "varifocal" or as a "variable focal length" lens, it should maintain focus as it is zoomed, at least to a degree that errors are insignificant.

The ability to focus at the long end and maintain focus while zooming to the short end of the range is not particularly useful with still photography, but it is the bread and butter of dramatic cinematography!

Most zooms are not perfectly parfocal. This is easy to established. The most precise test results being obtained using live view and manual focus. You cannot expect regular AF to be as accurate at the short end than it is at the long end: it is perfectly normal that a lens re-adjusts focus as you zoom in, since it gets a magnified view of the subject to focus on: same as with your own eyes. So, parfocality is better confirmed when zooming back . You do not even have to take actual shots: full zoom, focus on an easy repeatable target, then zoom fully back and re-focus: any refocussing can be observed on the distance scale.

To answer your question, it is best to focus with final framing OR focus at the long and zoom back if perfect parfocality is confirmed on that lens. The same goes for focus/reframe: at short distances, any reframing will definitely shift the focus plane visibly.
--
Jean Bernier

All photographs are only more or less credible illusions

Thank you for your informative and thoughtful comments. My move will be to see what Canon USA or Canon Europe have to say about their lenses.
Bob Hartman

Correction My next move will be to do some testing myself. One series at about 5 feet and another at 15 feet. Testing will be on a tripod without IS, with and w/o auto focus and w, w/o re-framing. See ya in a week - thanks again.
Bob Hartman

I never saw that feature being specified at Nikon. I'd be interested to know what they say at Canon.After all, AF corrects in real time these discrepancies, if focus is not locked. Use manual focus, and you could be scr* ed is you zoom later.
Just to give you an idea, my 14-24 Nikkor is not really parfocal
My 24-70 is for all practical purposes parfocal
My 70-200 VRII is nearly parfocal. It's predecessor was badly non-parfocal.
So I always re-focus after any zooming or re-framing.
--
Jean Bernier

All photographs are only more or less credible illusions

Frame your shot the way you want it, focus, and shoot.

Although using the center focus point might not be such a hot idea.

That's why there's lots of focus points on most modern cameras, and the ability to touch up focus manually on many lenses.

BAK

All of you make a lot of sense. Remembering which of my lenses are very good at parfocal is not in the cards for me. Cheers.
Bob Hartman

kbhartman said:

All of you make a lot of sense. Remembering which of my lenses are very good at parfocal is not in the cards for me. Cheers.
Bob Hartman

Click to expand...

It's no big deal, really. It takes just a few minutes per zoom to establish. Or maybe you have too many zooms? I found out the hard way with an older lens in the times, when a zoom perfectly focussed at the long end (manually) produced pictures visibly oof when zoomed back for the takes. Since the subject was static and I was tripoded, I thought I'd use the best technique...Never was aware of the problem for many months using that zoom, but on that day, the problem showed up...! I put a small sticker on that one, reading "non parfocal". End of problem.
Cheers.

--
Jean Bernier

All photographs are only more or less credible illusions

I used to focus only using center then recompose but now the 7D makes it very easy to compose using the zone focus mode.

However, on my 5dMii I still use the old method.

Raymond Bradlau said:

I am pretty sure most zooms are not true parfocal and do not hold there focus as you zoom in and out.

Click to expand...

I had never even heard the phrase parfocal before today and appreciate gaining some knowledge. Thanks to all the other posters who added supplemental information on the subject.

For me, I'll continue to zoom where I want, and then focus. But this has been a very interesting learning experience.

jean bernier said:

You do not even have to take actual shots: full zoom, focus on an easy repeatable target, then zoom fully back and re-focus: any refocussing can be observed on the distance scale.

Click to expand...

I'm not sure that's a good test. Suppose, for example, that you would like the point of critical focus to be on the subject's eyes. You zoom to the longest focal length of the lens (zoom in to maximum magnification) and focus. Then you zoom completely out to the short end of the focal range and hit the focus button again. At this point, your focus bracket/ focus area on the camera's AF system will have a much broader target than just the eyeball or eyeballs. The AF system will now seek the best compromise focus for whatever is in the focus area. That's likely to be different than a perfect focus on the eyes - maybe, for example, it includes the eyebrows, or if the zoom range is large, perhaps it includes the subject's hair as well. This second focus solution will be a property of the camera AF system, not just the lens. I don't think it proves whether the lens is parfocal. I think "parfocal" in this context ought to mean that the initial focus on the eyeballs will make the eyeballs as sharp as they can be once the lens is zoomed out, not that the lens/camera will not choose a different focus when presented with a larger area as the focus target.

To my way of thinking, a better test would be: Use a standard focus test target, camera on tripod. Use the center focus point. Zoom in as close as possible, focus on the target. Now zoom back to the shortest focal length and take a shot without re-focussing. Finally, without changing the focal length from the previous shot, focus again on the target and take another shot. Now import both shots on your computer and zoom in to 100% magnification. Is the first shot well in focus on the center of the focus target, where you actually focussed? Is the second shot in better focus on the center? If the answer to the first question is "Yes," and the answer to the second is "No," then for practical purposes, I'd say the lens is near enough to parfocal for practical purposes. If the second shot gives better focus on the center of the focus target, the lens is not parfocal for practical purposes.

What do you think?

Ray

Ray Ritchie said:

jean bernier said:

You do not even have to take actual shots: full zoom, focus on an easy repeatable target, then zoom fully back and re-focus: any refocussing can be observed on the distance scale.

Click to expand...

I'm not sure that's a good test. Suppose, for example, that you would like the point of critical focus to be on the subject's eyes. You zoom to the longest focal length of the lens (zoom in to maximum magnification) and focus. Then you zoom completely out to the short end of the focal range and hit the focus button again. At this point, your focus bracket/ focus area on the camera's AF system will have a much broader target than just the eyeball or eyeballs. The AF system will now seek the best compromise focus for whatever is in the focus area. That's likely to be different than a perfect focus on the eyes - maybe, for example, it includes the eyebrows, or if the zoom range is large, perhaps it includes the subject's hair as well. This second focus solution will be a property of the camera AF system, not just the lens. I don't think it proves whether the lens is parfocal. I think "parfocal" in this context ought to mean that the initial focus on the eyeballs will make the eyeballs as sharp as they can be once the lens is zoomed out, not that the lens/camera will not choose a different focus when presented with a larger area as the focus target.

Click to expand...

Well, my test cannot take into account the area covered by the focus areas: real shooting presents particular problems. If AF is used (or used to manual focus--green confirmation light), then you're totally dependant on the camera's focus array.

My test involves a flat target with say, a few easy to focus on feature. It's a lab kind of test in order to see if the lens maintains focus when zoomed back, period. What
the AF will do is rather unpredictable and irrelevant at this moment.

Ray Ritchie said:

To my way of thinking, a better test would be: Use a standard focus test target, camera on tripod. Use the center focus point. Zoom in as close as possible, focus on the target. Now zoom back to the shortest focal length and take a shot without re-focussing.

Click to expand...

That's exactly what I suggested. Or my description was not clear or you misread me.

You do not really have to take a shot (although it could more accurate to examine results on the lcd screen at high mag, not really worth downloading to the computer).

Just look at the distance scale on the lens and see if after zooming back, another press of the focus button (preferably single focus) constantly re-focusses the lens after zooming back .

Finally, without changing the focal length from the previous shot, focus again on the target and take another shot. Now import both shots on your computer and zoom in to 100% magnification. Is the first shot well in focus on the center of the focus target, where you actually focussed? Is the second shot in better focus on the center? If the answer to the first question is "Yes," and the answer to the second is "No," then for practical purposes, I'd say the lens is near enough to parfocal for practical purposes. If the second shot gives better focus on the center of the focus target, the lens is not parfocal for practical purposes.

Ray Ritchie said:

What do you think?

Click to expand...

I think we're talking the same, only in different words. An ideal zoom is parfocal, so that you do not have to worry about this. Bottom line is: beware of the "zoom-in for focus, zoom back for taking" technique if the properties of the lens are unknown.

As a side note, if you ever watched a tv cameraman work, he will constantly zoom in for precise focus then frame zoomed back. The lens is assumed or known to be parfocal. Anyway, the definition of regular tv was so poor, that the small discrepancies got usually undetected. The thing is most video electronic vf are of relatively low resolution, and small, so nailing focus when zoomed back was difficult. The final picture is not available to the cameraman, it is on a much larger screen...with the advent of HD TV, many cameramen started being blamed for oof images, the vf being regular lo-rez and the output being hi-rez (at least when introduced)...the dslr is today of such resolution that any error is plain visible, hence the need to know one's equipment.

Ray Ritchie said:

Ray

Click to expand...

--
Jean Bernier

All photographs are only more or less credible illusions

(I've removed my own words from your post above, because I think the colors of the text were somehow confusing my words and yours)

jean bernier said:

My test involves a flat target with say, a few easy to focus on feature. It's a lab kind of test in order to see if the lens maintains focus when zoomed back, period. What
the AF will do is rather unpredictable and irrelevant at this moment.

Click to expand...

I don't think what the AF does is irrelevant at all, since you specifically suggested that we zoom in, focus, zoom out and re-focus, and then check the distance scale for differences. The difference on the distance scale will definitely be impacted by the AF mechanism, since the camera will be trying to achieve the best average focus for the subject falling on the AF sensor - obviously, the subject that falls on the AF sensor is different depending on how far out you're zoomed.

jean bernier said:

That's exactly what I suggested. Or my description was not clear or you misread me.

You do not really have to take a shot (although it could more accurate to examine results on the lcd screen at high mag, not really worth downloading to the computer).

Just look at the distance scale on the lens and see if after zooming back, another press of the focus button (preferably single focus) constantly re-focusses the lens after zooming back.

Click to expand...

See my previous comment. I think you definitely should take a shot. Looking at the distance scale and whether it changes tells you only that the AF system decided it could do better, given that a different subject now falls on the AF sensor.

jean bernier said:

I think we're talking the same, only in different words. An ideal zoom is parfocal, so that you do not have to worry about this. Bottom line is: beware of the "zoom-in for focus, zoom back for taking" technique if the properties of the lens are unknown.

Click to expand...

I don't think we're talking the same, but I do agree we're probably not understanding each other.

jean bernier said:

As a side note, if you ever watched a tv cameraman work, he will constantly zoom in for precise focus then frame zoomed back. The lens is assumed or known to be parfocal. Anyway, the definition of regular tv was so poor, that the small discrepancies got usually undetected. The thing is most video electronic vf are of relatively low resolution, and small, so nailing focus when zoomed back was difficult. The final picture is not available to the cameraman, it is on a much larger screen...with the advent of HD TV, many cameramen started being blamed for oof images, the vf being regular lo-rez and the output being hi-rez (at least when introduced)...the dslr is today of such resolution that any error is plain visible, hence the need to know one's equipment.

Click to expand...

To test for actual parfocality of a lens, I think you'd have to come up with a test that completely takes the AF mechanism out of the equation. What I was suggesting was simply that the thing that matters to the photographer is whether the part of the image that he wants to be in best focus ends up in better focus by zooming in and focussing, then zooming out to shoot, or by simply framing as desired, focussing, and then shooting. The test I was suggesting, in which you shoot actual images (rather than just looking at the distance scale), was intended to answer that question.

I think it's also important to note that you don't always have the same focus objective. Sometimes when you shoot a full-body portrait of a model you might want the eyes in best focus. This might be almost impossible to attain reliably by using either AF or MF with the lens zoomed at the final framing, because the eyes are just too small a portion of the image to accurately tell whether they're in focus or not. But other times, with the same framing, you might want, say, the clothing the model is wearing to be in best focus (think catalog shot). In that case, using AF at the final zoomed distance may be the best approach. Or in the case of landscape photography, sometimes you'll want the foreground objects to be in critical focus, and sometimes you'll want a distance object to be in best focus, and let the foreground be slightly OOF. A different focus strategy might be dictated for each case.

I do agree that you have to test your own equipment and figure out how it works, rather than relying blindly on the AF, or assuming that the lens is parfocal. My disagreement seems to be mostly over what's the best way to test to get that understanding. I think you have to actually shoot images.

Ray

Ray Ritchie said:

(I've removed my own words from your post above, because I think the colors of the text were somehow confusing my words and yours)

jean bernier said:

My test involves a flat target with say, a few easy to focus on feature. It's a lab kind of test in order to see if the lens maintains focus when zoomed back, period. What
the AF will do is rather unpredictable and irrelevant at this moment.

Click to expand...

I don't think what the AF does is irrelevant at all, since you specifically suggested that we zoom in, focus, zoom out and re-focus, and then check the distance scale for differences. The difference on the distance scale will definitely be impacted by the AF mechanism, since the camera will be trying to achieve the best average focus for the subject falling on the AF sensor - obviously, the subject that falls on the AF sensor is different depending on how far out you're zoomed.

Click to expand...

Correct. I meant the features the AF system "see" is a separate issue. It is relevant in actual use if you choose to AF, wich most everybody will do...VFinders are not really accurate enough for easy manual focus at present day's resolutions...If a lens is not parfocal, you will probably get sharper pics if you AF at the final framing, even though the AF has a smaller view of the point of interest, than zooming in, focussing, lock focus and zoom back.
A parfocal zoom is a gem...I only have one.

Ray Ritchie said:

Ray Ritchie said:

That's exactly what I suggested. Or my description was not clear or you misread me.

You do not really have to take a shot (although it could more accurate to examine results on the lcd screen at high mag, not really worth downloading to the computer).

Just look at the distance scale on the lens and see if after zooming back, another press of the focus button (preferably single focus) constantly re-focusses the lens after zooming back.

Click to expand...

See my previous comment. I think you definitely should take a shot. Looking at the distance scale and whether it changes tells you only that the AF system decided it could do better, given that a different subject now falls on the AF sensor.

Click to expand...

Correct. I meant looking at the distance scale hints there is non-parfocality. It indicates too that you're probably best to AF at final framing, if the target is flat and unmistakable, such as a resolution target or something similar. To investigate further, you can open up live view at high magnification and manual focus at full zoom, then zoom back and observe the image on the lcd: is it still in optimum focus?
This is the ultimate way to test , obviously.

Taking pictures is not really required with live view IMO, it's up to you if you're more comfortable at the computer...

Ray Ritchie said:

Ray Ritchie said:

I think we're talking the same, only in different words. An ideal zoom is parfocal, so that you do not have to worry about this. Bottom line is: beware of the "zoom-in for focus, zoom back for taking" technique if the properties of the lens are unknown.

Click to expand...

I don't think we're talking the same, but I do agree we're probably not understanding each other.

To test for actual parfocality of a lens, I think you'd have to come up with a test that completely takes the AF mechanism out of the equation.

Click to expand...

Agreed. What I suggested is a crude test involving the AF. Let's say, that was step one.

Ray Ritchie said:

What I was suggesting was simply that the thing that matters to the photographer is whether the part of the image that he wants to be in best focus ends up in better focus by zooming in and focussing, then zooming out to shoot, or by simply framing as desired, focussing, and then shooting. The test I was suggesting, in which you shoot actual images (rather than just looking at the distance scale), was intended to answer that question.

I think it's also important to note that you don't always have the same focus objective. Sometimes when you shoot a full-body portrait of a model you might want the eyes in best focus. This might be almost impossible to attain reliably by using either AF or MF with the lens zoomed at the final framing, because the eyes are just too small a portion of the image to accurately tell whether they're in focus or not. But other times, with the same framing, you might want, say, the clothing the model is wearing to be in best focus (think catalog shot). In that case, using AF at the final zoomed distance may be the best approach. Or in the case of landscape photography, sometimes you'll want the foreground objects to be in critical focus, and sometimes you'll want a distance object to be in best focus, and let the foreground be slightly OOF. A different focus strategy might be dictated for each case.

I do agree that you have to test your own equipment and figure out how it works, rather than relying blindly on the AF, or assuming that the lens is parfocal.

Click to expand...

I could not say it better.

Ray Ritchie said:

My disagreement seems to be mostly over what's the best way to test to get that understanding. I think you have to actually shoot images.

Click to expand...

Civilized exchange of opinions. appreciated.
JB

Ray Ritchie said:

Ray

Click to expand...

--
Jean Bernier

All photographs are only more or less credible illusions