Now that we have the Event Horizon Telescope (EHT), why can't we just guess and check intergalactic distances by changing the EHT's focal length? For instance why don't we take another picture of M87, but assume it is actually 50 lightyears further away, and see if the image is any clearer? If so than M87 is actually closer to being 50 light years further away than it is where we currently assume. Try again enough times and we shouldn't we be able to determine the galaxy's position to the same precision we can change the focal point of the EHT? I assume the blurriness of the M87 picture was mostly due to the long exposure time, but it couldn't be the only factor. Obviously the depth of field increases for galaxies further away (if I'm using "depth of field" correctly), but I would think that measuring even the closest galaxies to such immense precision would render standard candles nearly obsolete, and cut out the bottom rungs of the cosmological distance ladder.
Why haven't I heard of using the EHT in this way?