[obsoquasi]

Hi guys, 


this one's a little tricky. I would like to do a series about oversized everyday objects. For example, a huge pencil lying on a normal sized desk. I will attempt to do this with two shots, one without the object and then one with only the object but enlarged. These two shots will then be composited in Photoshop using layers. 


Now the tricky bit. I can't just zoom in to make it larger, because that would change the perspective properties of the object. The normal sized object and the large version share a common set of vanishing points, which is not the case when you change the focal length. So I have to move my camera closer to the object. I must neither tilt nor pan the camera, because that again would mess up the location of the vanishing points. I am only allowed to shift the camera forward towards the object keeping the view orientation constant. This shouldn't actually be too hard. I can lock my camera on the tripod and make sure two of the tripods legs are in line with each other. My studio floor consists of wooden boards and the resulting stripes can be used as reference for the tripod legs. As long as I move the tripod with it's front legs in line, I'll be fine I think. 

 

And the last tricky bit: When my object is in the dead center of the photo and I move my camera closer to get this magnification effect, the object will grow in all three axis (x/y/z) equaly. If that object is lying on a table like in that pencil example, I must offset the object by shifting the camera down slightly. Things get even more difficult if the object is outside of the center of the frame, because it will (apparently) grow away from the image center, in that case I have to offset the camera both in the horizontal and in the vertical plane to put it back in the original position.

 

I am sure there is some kind of math to calculate all of that, but I think the easiest way would be to keep the camera tethered to a computer and then alternate between shifting the camera (forward, up and down, NO TILT OR PAN!!) and go back into Photoshop to see wether I have found the right offset yet to place the object on the desired sport on the background. I might even leave some kind of markers in the background where the magified object to we located at in the end. 

 

Maybe, just maybe I'd have to adjust the aperture as well, because the DOF will change because I am getting closer to the object and have to re-focus. But that shouldn't be too hard to figure out.

 

How about the change in lighting? Some of the lights might have to be readjusted, because this oversized object would be lit from slightly different angles then a normal shaped object. And if you want to get really picky, the apparent size of the lights would also change. Meaning if you make your object double the height, you have to decrease the size of the lights by half (e.g. 1x1m softbox should become a 50x50cm softbox). Especially relevant if the object to be magnified has a reflective surface. Otherwise the shadows will be too soft for an object of that size. 


That's how far I got. I was wondering if anybody else has ever tried to wrap his head around this subject and came up with a workable theory on how to approach it. 


Cheers, 


John

 

www.obsoquasi.ch | Facebook

[obsoquasi]

...

 

wrong forum maybe? I take it there aren't many photo designers here who would be interested in that sort of stuff. That's okay though, I thought I'd give it a shot . 


Cheers, 


John

 

 

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[Brightcolours]

You will HAVE to change the focal length. Else it will not work. 

 

This is why: the perspective changes with your distance. Test this with the table. If you look at the corner of a table at say 2 meters distance, and then look at the same corner with just 20cm distance, the angles will have increased.

 

I do not know what exactly the formula will be, for changing the focal length to keep the angles the same when you change your distance. But I think it is probably more simple than you think:

Most probably the only thing you have to do is... NOT change the distance to your subject(s). Only zoom in on the pencil by increasing focal length. This will keep the perspective correct, and will give you what you need. 

 

To keep the DOF similar, use the same diameters of aperture. So, lets say you are shooting the table with a 50mm normal prime, at f4. The aperture size will be: 50 / 4 = 12.5mm. And suppose you shoot the pencil with a 200mm lens. Then use a 12.5mm aperture too (for similar DOF). The f-value you need to set will be: 200 / 12.5 = f16.

 

So, to recap: Do NOT tilt/shift/move the camera in any way between shots. Keep the height the same too. Only change focal length to get the desired magnification. Use the same aperture size to keep DOF similar.

 

Quick and dirty example (handheld, no DOF correction) with 18mm (APS-C), then zoomed in to 50mm, then combined:



 

 

I do realize that this method is nor perfect either, as you might look on top of a small subject, but when it would be bigger you would look upto it. I will have to rake my brain a bit more over this. 

Maybe this solves that problem: Note where in the frame the subject rests in the table (the height of the frame). When zooming in, keep that height of that point the same (by lowering the tripod, not the camera angle).

 

Good luck with your experiments, and your thoughts on my thoughts are welcome.

[obsoquasi]

Hi Brightcolours, 


hey awesome, a fellow good soul who finds interest in this matter. It's quite a "delicious" little problem isn't it. Now the reason, why I think you can't change the focal lenght is that if an object grows in three-dimensional space, it's not just larger on the screen (as if you'd simply increase it's size in Photoshop), it grows away from the center point and gets more and more distorted. But the vanishing points remain the same! All the parallel lines of a dice for example, be it big or small, meet at the same point. This is not the case when you change the focal length, because "zooming in or out" affects the vanishing points equally. Increasing the focal lenght moves the vanishing points away from the image center (or maybe it's the object center, not sure, either way, they move outwards). I'll try and do some sort of test in the following weeks to confirm this. 


In the mean-time I've started a blog post about this and hope to expand it as I discover new facts.


http://obsoquasi.ch/photo-manipulation-o...d-objects/


Cheers, 


John

www.obsoquasi.ch | Facebook