Take pictures at real-world size without a 1:1 lens

[Bedo]

The standard method I already suggested will work for you. Now that you have explained more (why didn't you do that to begin with?) it is even simpler. You make a wire frame that holds the camera (with pre-focused lens) a fixed distance from the ear. First you use it to take a picture of a ruler. This tells you your real pixel dimension to use for later image pixel-distance conversion (ie your pixel to mm conversion). (Forget that equivalence you found somewhere - it isn't appropriate for you and you are simply confusing yourself with it.) Then you take pictures of the ears, framing them in the wires. Use the longest lens you have to limit errors caused by distance. This is all tried and tested practice.

Yes I think that I will try this approach! Thank you

The pixel size of 0.264mm is, I think, a pretty standard dot pitch for monitors at some standard resolutions. I assume from this that the OP is interested in rendering things in "real life" size on a monitor.

Maybe it is a pretty standard for 72ppi monitor but I tried to calculate it and obtain the same result, without any luck... Are you able to obtain this *magic dangerous value*? It is interesting...

The pixel size of 0.264mm is, I think, a pretty standard dot pitch for monitors at some standard resolutions. I assume from this that the OP is interested in rendering things in "real life" size on a monitor.

That's what I thought the first post sounded like (1:1 on the monitor). Now it sounds like all that is required is a simple way of quickly converting from image pixels to distance. The 0.26 mm thing is a red herring, and the sooner the OP realizes that the better. The quoted 9 sf precision is kinda worrying, however. Also strange is the absence of a response to the various methods already suggested.

I insisted on the 0.264mm because it has actually been used on a IEEE scientific publication, so I will definitely investigate about this.

Thank you a lot for the help I'm going to try/ask something and then let you know

 

[unpopular]

^ as I said, I'm sure 0.264mm/p is *something* I'm just not sure it's what you need.

 

[The_Traveler]

Here is an excellent procedural article on how to do anatomic measurements and correct for distortion in placement but it requires the ABFO-2 ruler.
Roger D Metcalf DDS JD D-ABFO - Bitemark Photography Tips 1 

 

[unpopular]

^^ which is cheap.

http://www.crimescene.com/store/index.php?main_page=product_info&products_id=342

I really think that's the best approach. since this is dealing with scientific imaging there's no aesthetic reason not to just use a scale.

 

[enzodm]

I insisted on the 0.264mm because it has actually been used on a IEEE scientific publication, so I will definitely investigate about this.

Thank you a lot for the help I'm going to try/ask something and then let you know

if you found that value in a scientific paper, it is because in their case this was their value -it's not like pi. If you have to show on your computer, you have to calculate your value (and however maybe will not be valid on your teacher computer). Measure the size of your monitor (e.g., on horizontal - pixels are not exactly squared, but for your needs you can approximate), divide for the number of pixels you are using (e.g., 1280), and that's all.

For the "difficult" part, Helen's method is the right way. If your exercise allows it, you might even apply a known marker close to the ear (e.g. an adhesive circle of known size). You may eve use ImageJ to calibrate the image size (Analyze/Set scale).

 

[EchoingWhisper]

What screen can fully show the elephant in life size a meter away?

 

[unpopular]

What screen can fully show the elephant in life size a meter away?

this screen should work.

 

[The_Traveler]

If I had to do a great many of these I would buy an additional metal lens hood and create a wife frame attached to that with a plexiglass with cutout for ear and the scale above cemented in place.
Lift the camera, place cutout over ear and shoot.
We used things like this for clinical stuff but I can't find an illustration of it.

 

[enzodm]

The ruler is needed in each picture only if something changes in the geometry (distance or focal length). If not, you may calibrate once. On the other side, if there is a ruler, there is no need for the wireframe thing, aimed at maintaining geometry.

 

[The_Traveler]

The ruler is needed in each picture only if something changes in the geometry (distance or focal length). If not, you may calibrate once. On the other side, if there is a ruler, there is no need for the wireframe thing, aimed at maintaining geometry.

I have found that the advantage of having a ruler (we didn't use abfo-2 because it wasn't yet available, just used a segment of plastic mm. rule) in every shot is that every individual shot, if necessary, can be verified as to geometry.

Our typical process was to pick a few samples at random, verify that the lens-subject distance was the same (we were using fixed focal length lenses on much more primitive cameras) and we were confident that the situation was stable. The plexiglass surround also was a convenient place to put a piece of tape with the subject number. Thus each slide (film) was self-verified and self-identified with no worries.

 

[Buckster]

This conversation has all been very interesting to me. I don't know that I have anything to really contribute here, but I thought I'd run an experiment on my own here to see what happens.

First, I used Photoshop to view the final image and, importantly, I used a view of "Print Size" for accuracy.

To prepare for that, I'll note that my monitor size is calibrated in Photoshop so that when I'm viewing something in "Print Size", 1" = 1" and can be confirmed on the Photoshop ruler. For those who don't know how to achieve that, it's pretty simple: Measure the physical screen area of your monitor. In my case, that's 23.5" wide by 13.25" tall. My screen resolution is 1920 X 1080. 1920 divided by 23.5 inches means that I have 81.7 pixels per inch on the horizontal measurement. 1080 divided by 13.25 means that I have 81.5 pixels per inch on the vertical measurement. In Photoshop, go to Edit > Preferences > Units & Rulers > Screen Resolution and plug in 81.5 (in my case). That completes the "Print Size" calibration in Photoshop.

Next, I took a photo of my ear with a ruler just below it, then brought it into Photoshop.

I straightened it using the edge of the ruler, then zoomed in and cropped it precisely at the inch markers that would include the whole ear. That made the image 3" wide, per the ruler visible in the shot.

So I resized the canvas to precisely 3" wide to properly size it.

Then I viewed it in "Print Size". When I held my real-world ruler up to the screen, sure enough, it matched the Photoshop ruler and the ruler in the photo - precisely 3" wide; The photo of my ear on the screen at that moment was 1:1 - "actual size".

Then, just for giggles, I printed it. Placing my ruler beneath the ruler in the print, they matched, 1:1 - both were 3" wide. The print of my ear was "actual size".

Don't know if that's at all useful, but it was both fun and enlightening on this end. :thumbup:

Here's the cropped picture of my ear that I used for the experiment:

Note that it won't be 1:1 on your screen unless your screen really IS 96 ppi, which is what the image was saved at for web viewing, AND your browser is displaying at 100% size.

 

[Bedo]

This conversation has all been very interesting to me. I don't know that I have anything to really contribute here, but I thought I'd run an experiment on my own here to see what happens.

First, I used Photoshop to view the final image and, importantly, I used a view of "Print Size" for accuracy.

To prepare for that, I'll note that my monitor size is calibrated in Photoshop so that when I'm viewing something in "Print Size", 1" = 1" and can be confirmed on the Photoshop ruler. For those who don't know how to achieve that, it's pretty simple: Measure the physical screen area of your monitor. In my case, that's 23.5" wide by 13.25" tall. My screen resolution is 1920 X 1080. 1920 divided by 23.5 inches means that I have 81.7 pixels per inch on the horizontal measurement. 1080 divided by 13.25 means that I have 81.5 pixels per inch on the vertical measurement. In Photoshop, go to Edit > Preferences > Units & Rulers > Screen Resolution and plug in 81.5 (in my case). That completes the "Print Size" calibration in Photoshop.

Next, I took a photo of my ear with a ruler just below it, then brought it into Photoshop.

I straightened it using the edge of the ruler, then zoomed in and cropped it precisely at the inch markers that would include the whole ear. That made the image 3" wide, per the ruler visible in the shot.

So I resized the canvas to precisely 3" wide to properly size it.

Then I viewed it in "Print Size". When I held my real-world ruler up to the screen, sure enough, it matched the Photoshop ruler and the ruler in the photo - precisely 3" wide; The photo of my ear on the screen at that moment was 1:1 - "actual size".

Then, just for giggles, I printed it. Placing my ruler beneath the ruler in the print, they matched, 1:1 - both were 3" wide. The print of my ear was "actual size".

Don't know if that's at all useful, but it was both fun and enlightening on this end. :thumbup:

Here's the cropped picture of my ear that I used for the experiment:

Note that it won't be 1:1 on your screen unless your screen really IS 96 ppi, which is what the image was saved at for web viewing, AND your browser is displaying at 100% size.

This is really interesting! Thank you a lot!

I've just measured my screen size and it looks like I have 93ppi. My measure is quite accurate

However some software (e.g. Matlab) reported that I have 96ppi! And Photoshop should display the ruler correctly when I set it at 96ppi, not 93ppi! I think I'll use 96ppi.

By the way I discovered that "magic conversion factor":

pixel to meters: 1/96 * 2.54 / 100 = 0,0002645

But this doesn't work in general.

See: matlab - convert pixel to cm - Stack Overflow

 

[Bedo]

The standard method I already suggested will work for you. Now that you have explained more (why didn't you do that to begin with?) it is even simpler. You make a wire frame that holds the camera (with pre-focused lens) a fixed distance from the ear. First you use it to take a picture of a ruler. This tells you your real pixel dimension to use for later image pixel-distance conversion (ie your pixel to mm conversion). (Forget that equivalence you found somewhere - it isn't appropriate for you and you are simply confusing yourself with it.) Then you take pictures of the ears, framing them in the wires. Use the longest lens you have to limit errors caused by distance. This is all tried and tested practice.

I think we will use this method!