Zoom bar focal lengths

Unfortunately the F707 does not give us the focal length of the zoom lens while composing and shooting a picture. This makes it difficult to reset the camera to a previous lens setting. (It does, however, give the focal length in the EXIF file, but only after downloading to a computer.) Even worse, the F707 resets the zoom to full wide (9.7mm) whenever you turn it on. (Interestingly, when you shut it off, the lens contracts to the full telephoto setting).

I had hoped the F707 used a stepper motor (a kind of digital moving motor) to move the lens assembly, thus offering the possibility to calculate the precise focal distance by measuring the steps. Unfortunately, Sony opted for a cheaper DC motor (you can see the analog movement while looking at the front of the lens while depressing the zoom button).

Fortunately the zoom bar offers some hope to calculate the focal length. If you notice that while depressing the zoom button, the marker in the zoom bar jumps in seemingly discrete steps. In fact there are precisely 15 steps within the zoom bar; 10 steps are for the actual zoom and 5 steps for the digital zoom. There are, of course, an infinite number of zoom positions between these jumps, but in actuality you can only stop in a finite number of them depending on how quickly you can tap the zoom button. (I can manage between 4 to 11 mini positions between each bar marker jump.)

I took 10 sets of photos for each jump position and wrote down the focal lengths. I then averaged the numbers to determine the median focal distance for every jump. The chart above shows the final average numbers.

The focal length numbers with the tolerances are as follows:

9.7 (+.0 -.0) Full wide
10.8 (+.1 -.1)
12.7 (+.1 -.2)
15.1 (+.2 -.2)
18.3 (+.5 -.2)
22.5 (+.3 -.3)
28.2 (+.3 -.5)
35.1 (+.5 -.4)
43.6 (+.5 -.6)
48.5 (+.0 -.0) Full telephoto

Notice that the tolerance error increases toward the Telephoto direction.

I also made measurements of each jump by starting from the 48.5 setting and going toward the 9.7 setting. The numbers are different! This is because there is a hysteresis error due to the DC motor movement. The F707 apparently uses the full wide position as the home position to reset the position. This perhaps explains why Sony opted to automatically set the lens to full wide whenever you turn on the camera.

The numbers did not come out as precisely as I would have liked, but perhaps this information might serve some use to someone. Also note that my numbers relate to only my camera. Because of unknown manufacturing tolerances, your camera may not agree with my numbers.

Jim

I understand why you want to do this but it would seem in the field you would get quite I headache trying to get in just the right spot.

Just as a note I rember that with the old Kodak 260 which was ahead of its time with auto rotation also let you set what zoom default the camera went to when it was turned on.

Another note, the Sony Pc100 mini DV used an odd zoom control. there is an electromagnet ring that controls the zoom, you can hear it shaking around when the camera is turned off. It works well but I don't know if they kept it with future minDV's

The type of photography I'm experimenting with increases the effective pixel density to 20 megapixels or more. This requires several shots with precise measurements from the subject to the camera. However, this gets tiring after a couple of shots and I'd like to take a rest and shut off the camera. But when I turn off the camera, I lose the zoom settings which destroys the entire shoot. If the F707 would give me the exact focal distance, I could reset the camera and continue. This would prevent me from getting further headaches.

David Goldwasser said:

I understand why you want to do this but it would seem in the field
you would get quite I headache trying to get in just the right spot.

Click to expand...

I am glad you did this, I was thinking about it, but never could summon the energy I am going to print it out for future use. Also the accuracy of the zoom when going to wide from telephoto I did not know about. That is good to know if you are going for accuracy start wide and go to the desired focal length. Good stuff, thanks again.

--
Shay

My Sony F707 Gallery: http://www.shaystephens.com/portfolio.asp

Thanks Shay, but before using the chart you might want to verify if the number averages agree with your camera. Due to my lack of knowledge of Sony's tolerance limits, I have no confidence that my numbers will agree with other F707s. I'd hate to hold responsibility for messing up one of your shots.

Jim

Shay Stephens said:

I am glad you did this, I was thinking about it, but never could
summon the energy I am going to print it out for future use.
Also the accuracy of the zoom when going to wide from telephoto I
did not know about. That is good to know if you are going for
accuracy start wide and go to the desired focal length. Good
stuff, thanks again.

--
Shay

My Sony F707 Gallery: http://www.shaystephens.com/portfolio.asp

Click to expand...

bah, the whole idea was to not have to do it myself hahaha. {sigh} Very well, it will be interesting to see how closely the two cameras will come.

--
Shay

My Sony F707 Gallery: http://www.shaystephens.com/portfolio.asp

I agree this is very good work, Jim. I hope Shay's results coincide closely with your's. Otherwise, we'll all have to do this, if we want the correct #'s for our cams (or not). I'm just happy with my new knowledge that the zoom has 10 stops (digital off).

Steve

Shay Stephens said:

bah, the whole idea was to not have to do it myself hahaha. {sigh}
Very well, it will be interesting to see how closely the two
cameras will come.

--
Shay

My Sony F707 Gallery: http://www.shaystephens.com/portfolio.asp

Click to expand...

--
http://www.pbase.com/slo2k

It sounds like maybe you are shooting some large flat object and moving the camera square to the wall and stitchingthings together? I could also be wrong. Don't know how else would get the 20mp.

If this is the case may be better off being far away from subject and use a panoramic head where the nodal point is set. You can rotate without any paralx and then use stitching software to merge together, and then you can correct image so it is orthagonal.

By the way this makes me think of another way to find zoom after the fact. Stiching software like Realviz Sticher can calcualte the FOV of the photo from two images, assuming you shot without paralax (using a panoramic head). To do this with top quality also have to account for barrel or pin cushion, so that would be tough when not at known zoom. I'm always using at full wide.

Ok I got much better consistancy by half pressing the zoom in very short taps. I did that (10 times each and averaged) for each optical zoom level and got the following:
10mm - 11mm - 13mm - 15mm - 18mm - 22mm - 28mm - 35mm - 44mm - 48mm

The key to getting the consistant results was starting from wide angle, and quick half press taps on the zoom button.

--
Shay

My Sony F707 Gallery: http://www.shaystephens.com/portfolio.asp

It's good you got similar numbers as I did. Thanks.

Jim

Shay Stephens said:

Ok I got much better consistancy by half pressing the zoom in very
short taps. I did that (10 times each and averaged) for each
optical zoom level and got the following:
10mm - 11mm - 13mm - 15mm - 18mm - 22mm - 28mm - 35mm - 44mm - 48mm
The key to getting the consistant results was starting from wide
angle, and quick half press taps on the zoom button.

--
Shay

My Sony F707 Gallery: http://www.shaystephens.com/portfolio.asp

Click to expand...

David Goldwasser said:

It sounds like maybe you are shooting some large flat object and
moving the camera square to the wall and stitchingthings together?
I could also be wrong. Don't know how else would get the 20mp.

Click to expand...

That's exactly what I'm doing.

David Goldwasser said:

If this is the case may be better off being far away from subject
and use a panoramic head where the nodal point is set. You can
rotate without any paralx and then use stitching software to merge
together, and then you can correct image so it is orthagonal.

Click to expand...

I'm shooting inside a room so this would not work. The shooting subject, at present, is a bookcase. The idea is to capture the entire bookcase and have every book title readable in the photo.

David Goldwasser said:

By the way this makes me think of another way to find zoom after
the fact. Stiching software like Realviz Sticher can calcualte the
FOV of the photo from two images, assuming you shot without paralax
(using a panoramic head). To do this with top quality also have to
account for barrel or pin cushion, so that would be tough when not
at known zoom. I'm always using at full wide.

Click to expand...

Barrel distortion is a problem but I'm using Panorama Tools to correct for this.

Jim