I've made a focus chart to test parfocal lenses.

[Bill Kerrigan]

Here’s a photo of a focus chart (ver 1.02) I’m making to check if my lenses are parfocal.
It uses my computer screen and a target taped at 45 degrees.

The system works, but I'd like to improve it... so I'm posting on a few forums.
If anyone with graphic skills wants to help... please write.

When I'm finished refining the chart, I’ll post a download here:
Lenses

Bill Kerrigan
bill@kerrigan.ca
http://www.kerrigan.ca

[Thomas Wong]

i have a Tamron 28-75mm F2.8 for pentax which is parfocal too
but my Sony 16-50 and Zeiss 24-70 are not, not sure if it's because of my LAEA2

[Bill Kerrigan]

Originally Posted by Thomas Wong

my Sony 16-50 and Zeiss 24-70 are not, not sure if it's because of my LAEA2

I think both those lenses are suppose to be a parfocal...
I'm not sure if mine are functioning properly.
That's why I'm making the chart...

[Thomas Wong]

i remember when i bought those lenses, it was parfocal. Then after using for awhile, it became non-parfocal.
Dunno why

[Bill Kerrigan]

Thomas, I'm glad you answered this post.
That's exactly what has happened to me!

I'll post my findings sometime next week.

Best,
Bill

[Alister Chapman]

I don't really see how any DSLR lens can be truly parfocal as non of them have any way to trim the back focus so any small variations in the spacing between the lens and sensor will upset the focus tracking.

[Bill Kerrigan]

Originally Posted by Alister Chapman

I don't really see how any DSLR lens can be truly parfocal as non of them have any way to trim the back focus so any small variations in the spacing between the lens and sensor will upset the focus tracking.

Your right Alister… maybe be we should adopt the term 'parfocalish'.

I used a LA-EA1 with a 24-70mm 2.8 Nikon, when I first bought the FS100.
A wonderful lens… but when I tested it with a focus chart, I could see it's sharpness weave in and out as I zoomed back.

I replaced the Nikon with a 24-70mm 2.8 Zeiss… which appeared 'parfocalish' when I tested it in the store.
Focusing at F2.8 - 4 now became easier.

But, recently I've been finding the Zeiss more difficult to keep in focus.
Possible reasons for this are:
(a) the lens needs realignment
(b) I've replaced the LA-EA1 with the LA-EA2
(c) I'm getting older

My testing system as been rather primitive…
I was about to buy a LensAlign MkII (Focus Calibration System) for $159.95 plus shipping… when it occurred to me, I might be able to do the same thing using the length of my 60cm iMac screen.

I'll shoot a test on Sunday and post my results.

[Alister Chapman]

A tape measure or ruler extended away from the lens also works, but a large screen TV or monitor also works as you'll see the moire shifting across the screen as the focus shifts.

[Leonard Levy]

I've been using a down-loadable lens resolution chart for years to check par focal qualities. You simply go to Kinkos and blow it up to 4 feet or so and mount it on a piece of foam core.Bigger the better. I posted it here on the web for other people to use:

http://www.leonardlevy.net/ISO_12233-reschart.pdf

My experience is that many of the Nikon zooms are par focal or so close that you can't focus it any better by eye when its wide anyway. That includes the 28-70, 17-35, 17-55, & 80-200.I think its also true for the 24-70 I did have to adjust the backfocus on my Sony F3 however so that it was in the right spot for Nikon with my Optitek adapter.

One additional problem with getting these zooms to be parfocal is that even if the optics are parfocal often the tolerances on the adapters for video cameras are not precise enough and most cameras don't have a backfocus adjustment. Speedboster does but I don't know how the additional optics might come in to play there.

This is tne reason I have been reluctant to switch to EF glass or anybody else for video. I just wish that someone would make an adapter to transmit power to those lenses so you could activate VR (same as IS) for use on an F3, FS100 or anything else out there. Much better choices for cinema than EF also because you have mechanical irises.

[Christopher Young]

I found that the Sony DT 16-50 is parfocal on the Sony SLT-A77 APSC DSLR but it is not when mated up with the LEA2 adapter on an FS700. The LEA2 is too short to achieve infinity focus at infinity; it will allow the lens to focus past infinity. Because it’s too short for correct infinity focus the lens is unable to maintain focus tracking throughout its zoom range. I had a look at shimming the LEA2 and it is a very finicky tricky mount to shim up due to the autofocus and iris mechanism so I gave it a miss.

If the 16-50 is mated up with a manual adapter which can easily be shimmed you can then definitely get it focusing at infinity and tracking focus the whole way through the zoom range. What is good with the Alpha lenses unlike Canon EF lenses is that they still have a mechanical actuating pin to control the iris so if you use an adapter that has an inbuilt pseudo ‘iris’ ring that engages that pin you can have full manual iris along with manual focus.

A while back I did a few quick tests one afternoon looking for good short constant aperture parfocal zoom lenses to suit the FS700. Lenses I could use for interviews where I could quickly reframe and not have to worry about refocusing and adjusting aperture. So far all the lenses I tested are working extremely well for that purpose. An old Minolta Rokkor MD 35-70mm has become one of my favourite interview lenses. It’s a good sharp, even wide open, low flare lens with good contrast. Decent examples can be found on eBay for around the $60-$75 mark.

The results of those tests can be seen here:

Sony FS700 Parfocal lens tests x 3 with and without ' speed booster' - 1080-50p down to 720-25p 10.June.13 on Vimeo

Chris Young
CYV Productions
Sydney