Many OT prophets claimed to speak with God face-to-face. New Testament Apostles John and Paul say that "no man hath seen God" except through Jesus Christ. Many mainstream Evangelicals interpret this to mean that God cannot be seen and has never been seen. Yet the disciple Stephen, Joseph Smith and John the Revelator all clearly tell us that they received a vision of both God the Father and God the Son. So how are we to understand John 1:18?
I think many of the prophets and apostles like Adam (after the fall), Stephen, John, Joseph Smith, who claimed to have seen God the Father in fact saw the "person" of Heavenly Father. But thinking of the temple, I think it may be that the face of Heavenly Father was still veiled in glory. This veil of glory would continue until man is perfected. This would explain all the "no one has seen" vs. "has seen" scriptures and the emphasis that Christ is the express image of the Father. In any event, the important point here is none of the prophets have seen any of it without first receiving the Holy Ghost and bearing witness of the Son of God.
Who is the image of the invisible God, the firstborn of every creature: (New Testament, Colossians 1:15)
Who being the brightness of his glory, and the express image of his person, and upholding all things by the word of his power, when he had by himself purged our sins, sat down on the right hand of the Majesty on high; (New Testament, Hebrews 1:3)
And being thus overcome with the Spirit, he was carried away in a vision, even that he saw the heavens open, and he thought he saw God sitting upon his throne, surrounded with numberless concourses of angels in the attitude of singing and praising their God. (Book of Mormon, 1 Nephi 1:8)
Lehi sees the the face of the "One" or the Son but only "thinks he sees" the Father on his thrown. There is a suggestion of glory in these cases veiling a clear view. Prophets are seeing the person of the Father but maybe not His face.
And immediately I was in the spirit: and, behold, a throne was set in heaven, and one sat on the throne. (New Testament, Revelation, Revelation 4:2)
John says that God was like Jasper and a Sadine stone to look upon. Jasper was the first stone in the Breastplate of Aaron and Sardine was the last stone suggesting a combination of all colors, or white light or great glory.
But he, being full of the Holy Ghost, looked up steadfastly into heaven, and saw the glory of God, and Jesus standing on the right hand of God, (New Testament, Acts 7:55)
Stephen emphasizes seeing the glory of the Father. But those prophets who spoke with God "face-to-face" were referring to Jehovah (God the Son) and not Eloheim (God the Father). Again, veil of glory or not, the overarching lesson here is that no one has seen the Father without first being quickened by the Holy Ghost and also bearing witness of the Son of God who is Jesus Christ.