It is easier to explain my question just by a sample photo from Lyndsey Adler:
See the models are so big in the photo — and still that big RV is also in the image. What focal length do I need for a photo like this?
With the lenses I have, if I use my 14-24mm lens, I can fit the background in the frame but the model will look so tiny, If I get closer to make the model later then it will look funny and distorted a lot. If I use my 70-200 mm range, I can't get both in the frame like she has done.
Using some reasonable assumptions, geometry, and a bit of math, we can put actual constraints on the composition.
Rearranging the equation in #5, we get: DT = DM * (5 HT / HM).
Now, let's make 2 more assumptions:
Plugging those numbers into our rearranged equation, we get that the trailer is about 5.5 times farther from the camera than the models are.
Okay, we have relative distances now, but what about actual distances? Those are dependent upon focal length. Again using similar triangles, the ratio of sensor height (HS) to focal length (ƒ) should be the same as model height to distance to the models (again, using the assumption that the models fill the entire frame height). In maths terms: HS / ƒ = HM / DM. Rearranging to solve for DM, we get: DM = ƒ * HM / HS
Assuming a full-frame camera, the sensor height HS = 24 mm. Picking a focal length of, say, 70 mm, we should be about 4.8 m (16 ft) from the models, and the trailer should be about 26 m (87 ft) away.
I tried searching for the make of the trailer in the picture to get actual dimensions. While I couldn't identify the make or model of the trailer, with some lucky Googling, I managed to identify the actual trailer used in the photo. This trailer is located here. Using the excellent The Photographer's Transit application on my phone, here are approximate shot layouts at 100mm and at 70mm (on a full-frame camera).
Using very rough estimation, I set up the shot so that the trailer is slightly less than half the width of the field of view, just at the left edge of the field of view.
Based on the cars/debris at the edge of the field of view, I think Lyndsey Adler's shot was less than 100mm.
Note that errors in the estimations we made (especially in eyeballing the camera and subject placement in Photo Transit) can have a big impact on the exact final comparison; however, this rough back-of-the-envelope calculation shows how some simple geometry and maths can yield a decent starting point in setting up your composition. (And, also, with some Google-fu and maybe blind luck, you can actually decompose a researched composition).
What focal length do I need for a photo like this?
It's not so much the focal length as the relative distances to the models and the background. I'd guess you could take a similar shot with a 35mm lens, a 50mm lens, a 100mm lens, or something even longer. Find a shooting position and focal length that lets you frame the background the way you want it, and then figure out where the models need to stand. Since the models are relatively close and the background is far away, you should be able to move around a bit to get the models framed just right without having a big impact on the background.
If I use my 70-200 mm range, I can't get both in the frame like she has done.
Sure you can... just back away from the models, or have the models back away from the camera, until they fit the way you want. The top of that trailer is probably 10 or more feet off the ground, right? And yet the trailer is maybe 1/4 the height of the models in the photo, because it's farther away. Use that same principle to adjust the height of the models as necessary to get the shot.
For my own POV.. This is achievable with several lenses.. you just need to properly position the distance of your camera from the model and your model from the background.
If I will be shooting this exact photo. I would use an 85mm prime lens with my iso set to 100-400max. Then my aperture will be at f/6.5-f/8 to ensure the sharpness and details of the model and the dress but still maintaining the blurred background with the lens's perfect focal length. In the fashion industry, our most used lens focal length is 85mm.
If you have doubts about lighting your models in the given aperture opening there a several ways to do it that you won't even notice that it is artificial lighting.
Adjust the distance between model and background.
The image was most likely not shot with anything <28mm. I'd start with 35 or 50mm and as said:Adjust the distance between model and background until things fit...
It could well be separately taken shots that are merged in Photoshop, but my guess is a long lense (200+ mm) and just the right distance from the models. You could walk back until your background fits, and the have the models walk forward until you get the shot you want. After some tries you will know where to position them.
It would be easy if someone goes there ;-)
The mountain is twice as high (in angular terms) as the trailer and I think the photo was taken from slightly more than the womens' knees.
From that place we would have to put a lens that crops the way we see in the photo. The models should be place so that they fill vertically the whole image.
My guess is it was taken between 35mm and 85mm lens on a FF. I guess 50mm.
The mountain in the background seems like a huge mountain but in reality is very small ;-)
